Many varieties of ON/OFF control circuits use some form of flip-flop that responds to a pushbutton switch or other control input. These all have volatile memory and default to an OFF condition if power is turned off, and sometimes that's even the preferred situation. However, if your application must remember what it was doing when power failed and resume from where it left off when power is restored, you may have a problem.

Here's a flip-flop circuit that remembers its state indefinitely when power is off. Furthermore, the circuit consumes no power at all, except for a brief instant when triggered from one state to the other. Thus, it’s well-suited to battery power; a couple of lithium coin cells can provide years of battery life.

In the design (Fig. 1), K1 is a 5-V dc, dual-coil magnetic latching relay with DPDT contacts. If you search the various distributor offerings, there are about 70 different models of this type relay at coil voltages from 4.5 to 24 V dc, made by four manufacturers, and costing between $3 and $8 in single quantities. They are well built, sealed, very small, and typically have contacts rated for 2 A at up to 250 V ac.

1. This two-coil relay configuration provides a latching flip-flop action without active electronics and retains its state even after power is removed. Component values are not critical.

One set of contacts (pins 2, 3, and 4) are used to steer the flip-flop, and the other set (7, 8, and 9) are available for the end application. In the state shown, C1 has charged to 5 V through R1. Closing S1 discharges C1 through D1 to pulse the K1A coil, transferring the contacts. Then C2 charges through R1 to await the next closure of S1, which discharges C2 through D2 to pulse the K1B coil, returning the contacts to the original state.

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Many varieties of ON/OFF control circuits use some form of flip-flop that responds to a pushbutton switch or other control input. These all have volatile memory and default to an OFF condition if power is turned off, and sometimes that's even the preferred situation. However, if your application must remember what it was doing when power failed and resume from where it left off when power is restored, you may have a problem.

Here's a flip-flop circuit that remembers its state indefinitely when power is off. Furthermore, the circuit consumes no power at all, except for a brief instant when triggered from one state to the other. Thus, it’s well-suited to battery power; a couple of lithium coin cells can provide years of battery life.

In the design (Fig. 1), K1 is a 5-V dc, dual-coil magnetic latching relay with DPDT contacts. If you search the various distributor offerings, there are about 70 different models of this type relay at coil voltages from 4.5 to 24 V dc, made by four manufacturers, and costing between $3 and $8 in single quantities. They are well built, sealed, very small, and typically have contacts rated for 2 A at up to 250 V ac.

1. This two-coil relay configuration provides a latching flip-flop action without active electronics and retains its state even after power is removed. Component values are not critical.

One set of contacts (pins 2, 3, and 4) are used to steer the flip-flop, and the other set (7, 8, and 9) are available for the end application. In the state shown, C1 has charged to 5 V through R1. Closing S1 discharges C1 through D1 to pulse the K1A coil, transferring the contacts. Then C2 charges through R1 to await the next closure of S1, which discharges C2 through D2 to pulse the K1B coil, returning the contacts to the original state.

This is not a high-speed circuit because of the setup time following a toggle. It was designed primarily for manual input and can toggle at least two times per second. Since power is applied to relay coils only briefly, the circuit tolerates overvoltage without harm, limited primarily by the capacitor voltage rating. When operated at 12 V dc, the circuit shown can toggle as fast as you can tap switch S1. On the other hand, if your application needs to restrict how fast something can be switched on and off, that can be easily limited by increasing the value of R1.

Component values are not very critical. C1 and C2 must store enough energy to pulse the relay, which has an operate time of 20 ms. R1 should be large enough that holding S1 closed does not allow capacitor voltage to rise above 10-20% of relay pull-in voltage.

2. This modified version of the original circuit uses a larger center-tapped relay coil with four times the contact-current rating of the two-coil relay approach.

 If you need more switching power than 2 A, Figure 2 shows the circuit adapted to a relay that’s four times bulkier and costs twice as much, but has contacts rated for 8 A at 250 V ac. This relay has a center-tapped coil instead of two separate coils, so the pin-out is different and the circuit must be modified slightly.

Tommy Tyler has a degree in mechanical engineering, cum laude, from Vanderbilt University. He retired 22 years ago after a career spanning over 40 years in the design of industrial instrumentation, medical electronics, consumer electronics, and robotics products, earning 17 patents in these fields. In his retirement, he still pursues his hobbies of electronic tinkering as well as technical writing and illustrating.

A popular power source for portable devices is a single lithium-ion (Li+) cell battery, which provides 4.2 V at full charge and 2.8 V near the end of discharge. Some functions within portable electron­ics, such as RF amplifier circuits for wireless communication and common microcontrollers, require 2.8- and 3.3-V input power rails.

These rails are normally provided by low-noise linear-dropout regulators (LDOs) to ensure a quiet supply. The LDO’s inputs (V_CC in Fig. 1) must be at a slightly higher voltage than the highest LDO output. Hence, V_CC ends up right in the middle of the Li+ battery’s range of operation. The use of a step-up/down voltage regulator, capable of operating from an input (V_BATT) that can be higher or lower than the output, becomes a necessity.

1. The LDO’s inputs must be at a slightly higher voltage than the highest LDO output.

In portable applications, the voltage-regulator efficiency is of the utmost importance since higher efficiency translates into longer untethered operation. This article reviews the available options, compares their performance, and presents a one-two punch approach to the most efficient solution.

Bypass-Boost

One way to solve the battery-to-LDO voltage mismatch is to use a bypass-boost converter, namely a boost converter with an extra “pass” transistor integrated between the power source (V_BATT) and the LDO input (V_CC). Figure 2 shows the bypass-boost powertrain architecture and its operation table. Here, the bypass transistor T₃ accomplishes a “poor man’s” step-down operation.

2. The bypass-boost powertrain architecture and its operation table highlight a “poor man’s” step-down operation.

This architecture can only regulate V_BATT voltages lower than the set V_CC = 3.4 V. For V_BATT> 3.4 V, the boost converter stops regu­lating and the pass transistor turns on, directly connecting V_BATT to V_CC. For most of the time (V_BATT> 3.4 V), the pass transistor in the bypass-boost architecture literally “passes the buck” to the LDOs downstream. The LDOs bear the task of regulating the high V_BATT value down to their output set values. Since this regulation is linear, the result is high power dissipation inside the LDO. The higher heat generated becomes a burden on the PCB in terms of cost, size, and reliability.

Buck-Boost

In contrast to the bypass-boost architecture, a buck-boost converter used in this circuit will never stop regulating its output to 3.4 V. In addition, the regulation is entirely switch mode, which provides high-efficiency operation. Figure 3 shows the buck-boost powertrain architecture and its operation table.

3. In contrast to Fig. 2, this uses a buck-boost powertrain.

For V_BATT> V_CC, the IC regulates in buck (step-down) mode, while for V_BATT< V_CC, it seamlessly transitions to boost (step-up) operation. This ensures that the V_CC output remains in regulation and is glitch-free. The entire battery voltage range is covered in a switch-mode, high-efficiency fashion.

Buck-Boost vs. Bypass-Boost

We compare the system efficiency (from V_BATT to V_OUT) using Maxim’sMAX77816 buck-boost IC versus a competitive bypass-boost IC (Fig. 4). Each step-up/down converter feeds a single 3.3-V LDO loaded with 500 mA.

4. This efficiency test setup uses a Maxim’s MAX77816 buck-boost IC or a competitive bypass-boost IC for the step up/down block.

Figure 5 shows the result of the comparison. Solid curves indicate efficiency and dashed curves show battery current consumption for each solution. The buck-boost efficiency (above 93% across the entire operation range) is far superior to that of the bypass-boost (as low as 81% with full battery). This superior performance is due to the ability of the buck-boost IC to supply power to the LDO in switch mode across the entire range of operation. The vertical dotted line highlights the transition point from step-up/down to step-down/up mode.

5. The buck-boost efficiency is above 93% across the entire operation range, making it far superior to that of the bypass-boost.

Buck-Boost vs. Buck-Boost

In Figure 6, we compare the efficiency (from V_BATT to V_CC) of MAX77816 to a similar buck-boost IC. The comparison in this case uses V_CC  = 3.3 V and V_BATT = 3.3 V, since the competing data is readily available, as opposed to V_CC = 3.4 V.

6. These results are based on V_CC  = 3.3 V and V_BATT = 3.3 V, since the competing data isn’t available for V_CC = 3.4 V.

Test results show that the MAX77816 outperforms the competing buck-boost over the entire current range from 1 mA to 3 A. The efficiency advantage is as high as 5%. This leads to the “first punch” of the solution to the efficiency problem: Use the best available buck-boost converter.

DVS for Efficiency

Dynamic voltage scaling (DVS) can further improve system efficiency. The buck-boost load is normally comprised of many LDOs, all at different V_OUT. These LDOs may not all be operational at the same time. When the LDO with the highest V_OUT is disabled, the system can lower the buck-boost output (V_CC) in a manner compatible with the next highest V_OUT, effectively reducing the voltage dropout and thus saving power.

DVS With Direct Hardware Control

A two-level output voltage selection is easily achieved with a dedicated DVS logic-input pin when direct hardware control is desired. The MAX77816 supports a programmable general-purpose input pin that can be configured as a DVS input between two preset/programmable values. The default values are 3.4 V and 5 V, but can be adjusted upon request. For further power savings, a more granular selection of output voltages must be implemented, which requires a different type of output voltage control. This is discussed in the next section.

The Advantage of an I²C-Driven DVS

The MAX77816 features an I²C-compatible serial interface, used for regulator on/off control, setting output voltage value and slew rates, and additional functions. With I²C, the regulator output voltage can be dynamically adjusted. This, in turn, enables finer control of system power consumption without the need for a dedicated DVS input pin.

The curves in Figure 7 refer to a Li+ battery-powered, step-up/down converter loaded with a 3-V-output, 32-mA LDO, and a second 2.85-V-output, 18-mA LDO. The descending orange curve is the discharge profile of the Li+ battery with typical system shutdown at 3.4 V (systems seldom deplete the battery all the way down to 2.8 V). The rest of the curves profile the accumulated extended battery operating time of the MAX77816 due to voltage scaling.

7. The descending orange curve is the discharge profile of the Li+ battery with typical system shutdown at 3.4 V. The rest of the curves profile the accumulated extended battery operating time of the system due to voltage scaling.

The light-blue curve shows the advantage of the buck-boost with 3.4-V output versus a boost-bypass architecture. The rest of the curves show the advantage of reducing the buck-boost output V_CC via I²C down to 3.15 V for duty cycles from 25% to 75%.

The extended battery operating time varies from 45 to 82 minutes. The second punch of the solution is clear: Employ DVS via an I²C bus.

Conclusion

A comparison of the buck-boost architecture to the by­pass-boost architecture shows that, in principle, the buck-boost is a superior architecture. A practical comparison of the MAX77816 buck-boost solution versus a competing bypass-boost solution shows that in operation, the MAX77816 has an efficiency advantage of up to 13%. When compared to a competing buck-boost solution, the MAX77816 outperforms it by as much as 6 efficiency points.

These efficiencies, combined with those derived from dynamic voltage scaling via an I²C bus or via a dedicated DVS input pin, result in extended operation for a battery-powered device of up to 90 minutes. Thus, a buck-boost IC with DVS is the ideal solution for power-stingy portable applications.

The Industrial Internet of Things (IoT), with its emphasis on real-time monitoring and control, has led to an explosion in the number of sensors attached to industrial machines and processes (Fig. 1). Many of the quantities being measured are analog in nature, including pressure, temperature, and flow.

1. The automated factory depends on accurate data from multiple analog sensors. (Source: TI blog: “How to ensure precision in automated processes”)

A typical data-acquisition system consists of a precision analog front end connected to the sensor, followed by an analog-to-digital converter that sends information over a wired or wireless connection to a gateway and then to the cloud.

Switching vs. Linear Regulators—A (Very) Short Overview

The power supply to an industrial data-acquisition system typically consists of a mix of switching and linear regulators. Each type of design has its strengths and weaknesses. At its heart, a switching design relies on a power transistor that changes the analog input voltage into a pulse-width-modulated (PWM) pulse train whose duty cycle depends on the output voltage and current. The topology chosen depends on the application. A switching regulator can convert an input voltage to a higher level (boost), a lower level (buck), or even convert a positive voltage to a negative voltage.

Switching regulators are highly efficient—up to 95% or higher—so a compact design can handle large amounts of power. The high efficiency makes a switching regulator the preferred choice for larger power-conversion tasks, such as providing an industrial rack with system dc power. To address that space, Texas Instruments has developed a wide array of switching regulators. 

Download this article in PDF format.

The Industrial Internet of Things (IoT), with its emphasis on real-time monitoring and control, has led to an explosion in the number of sensors attached to industrial machines and processes (Fig. 1). Many of the quantities being measured are analog in nature, including pressure, temperature, and flow.

1. The automated factory depends on accurate data from multiple analog sensors. (Source: TI blog: “How to ensure precision in automated processes”)

A typical data-acquisition system consists of a precision analog front end connected to the sensor, followed by an analog-to-digital converter that sends information over a wired or wireless connection to a gateway and then to the cloud.

 Sponsored Resources: 

• Linear Power for automated industrial systems
• How hot is your LDO?
• LDO basics: Current limit

Switching vs. Linear Regulators—A (Very) Short Overview

The power supply to an industrial data-acquisition system typically consists of a mix of switching and linear regulators. Each type of design has its strengths and weaknesses. At its heart, a switching design relies on a power transistor that changes the analog input voltage into a pulse-width-modulated (PWM) pulse train whose duty cycle depends on the output voltage and current. The topology chosen depends on the application. A switching regulator can convert an input voltage to a higher level (boost), a lower level (buck), or even convert a positive voltage to a negative voltage.

Switching regulators are highly efficient—up to 95% or higher—so a compact design can handle large amounts of power. The high efficiency makes a switching regulator the preferred choice for larger power-conversion tasks, such as providing an industrial rack with system dc power. To address that space, Texas Instruments has developed a wide array of switching regulators. 

On the other hand, the transistor’s switching action generates noise that appears as a ripple on the output. In addition, there’s some amount of delay before the switching controller can detect and respond to sudden load changes.

In a data-acquisition system, the high level of noise is a problem when providing clean power to the sensitive analog circuitry that buffers or amplifies a low-level sensor input. That’s the domain of the linear regulator.

2. A linear regulator maintains a constant voltage V_OUT as the load current varies. The PNP pass transistor dissipates excess power as heat. (Source: TI: “Linear power for automated industrial systems” PDF)

A linear regulator (Fig. 2) has very low output ripple and noise because it doesn’t use any switching element. Instead, its power transistor operates continuously. Any difference between the desired output voltage and the actual one is expressed as an error signal. The power-transistor control circuit uses the error signal to adjust the power transistor and move the output voltage closer to the desired value. The feedback loop is analog in nature, so the linear regulator responds instantaneously to load variations.

A linear design can only convert a higher input voltage V_IN to a lower output voltage V_OUT. A voltage representing the difference between the input and output (V_IN− V_OUT) appears across the power transistor; for a current I_OUT, the power transistor therefore dissipates the wasted power as heat:

P_D = (V_IN− V_OUT) × I_OUT + (V_IN× I_ground)

(V_IN x I_ground) represents the power consumed by the control circuitry of the device. As (V_IN− V_OUT) increases, the linear regulator dissipates more power and the power supply becomes progressively less efficient. Therefore, the difference between V_IN and V_OUT should be kept as low as possible.

A linear regulator’s input voltage must be higher than the desired output voltage by a certain value—the dropout voltage V_DO—to maintain regulation. If the input falls below the minimum value, the output voltage begins to decline. The value of V_DO varies with input voltage, output current, and junction temperature, but a low-dropout (LDO) regulator can have a dropout voltage as low as a few tenths of a volt. Key LDO parameters include dropout voltage, output voltage, output current, input voltage range, package type, package size, control features like enable or soft-start, power-dissipation capability, and noise performance.

Texas Instruments offers over 500 LDO regulators for industrial, consumer, communications, and automotive applications.

LDO Thermal Performance and Packaging

Since the LDO dissipates excess power as heat, its thermal performance is of great interest to the designer. There are two main areas of concern: Making sure that the part doesn’t get too hot; and protecting it if the temperature exceeds a safe threshold. Let’s take a closer look at these two topics.

Why do we care about the temperature of an LDO? Although low temperatures can cause problems, in power devices we’re primarily concerned about high temperatures.

In fact, at a high-enough temperature (around 290°C for doped silicon), semiconductor action ceases—the electrical differences between the n- and p-regions disappear, and the p-n junction no longer controls the carrier flow. Long before that point, though, thermal overstress caused by excess heat melts the package, warping and cracking the integrated circuit.

There are more subtle effects, too. Many operating parameters are temperature-dependent. At high temperatures, the device may still operate, but compliance with the datasheet specifications is no longer guaranteed.

An LDO datasheet lists several high-temperature limits, as well as the likely consequences if they’re exceeded. Consult the datasheet for specifics, but the table below summarizes the results.

During operation, the LDO temperature at the junction T_J rises above ambient temperature T_A due to the power P_D dissipated across the pass transistor. The value of T_J is given by:

T_J = T_A + (P_D x R_θJA)

where R_θJA is the thermal resistance of the device from the junction to the ambient environment, expressed as degrees Celsius per watt. This parameter is stated in the datasheet as shown in Figure 3. It specifies the temperature rise for each watt of power consumed and is a measure of the thermal performance of the device package. You should be careful not to rely solely on R_θJA to estimate the temperature of the device in the application, because the real-world value also depends on the PCB design, layout, and other factors.

3. Many LDOs, such as the TPS759 shown here, offer a choice of packages with different R_θJA values. Note that the test conditions are clearly stated. (Source: TI: “Power Good Fast-Transient Response 7.5-A Low-Dropout Voltage Regulators” PDF)

You can compare the thermal performance of two devices from different manufacturers if each one uses a standardized test, such as JEDEC’s EIA/JESD51-x standard, to measure R_θJA. For more information on the thermal performance of IC packages, consult this application report.

Follow These Steps to Minimize LDO Temperature Rise

If an LDO is supplying power, its temperature is going to rise. But you can take steps to help remove the heat from the device and battle the laws of physics.

Many LDOs come in a variety of packages. Unless your design is severely space-limited, you can choose a larger, more thermally efficient package. Choosing the right package can make a big difference. The TPS732 250-mA LDO, for example, is available in three package types: an 8-pin SON (3.00 × 3.00 mm), a 6-pin SOT-223 (6.50 × 3.50 mm), and a 5-pin SOT-23 (2.90 × 1.60 mm).

The values of R_θJA for these packages are 58.3, 53.1, and 205.9°C/W respectively. Why the huge variation? The SON and SOT-223 are relatively large power packages with exposed copper pads; these are soldered to the PCB ground plane and provide a thermally efficient conduit to remove heat. The SOT-23 package lacks a thermal pad; it's also the smallest in size, so it has the smallest surface area to remove heat through radiation and convection.

We can perform a quick calculation to illustrate the difference between packages in the TPS732. Let’s assume V_IN = 5.5 V, V_OUT = 3 V, and I_OUT = 250 mA. The ground current I_ground varies with temperature, V_IN, and I_OUT; at the maximum recommended operating temperature, 125°C, it’s about 0.72 mA.

Using the equation above gives:

P_D = (5.5 – 3.0) × 0.25 + (5.5 × 0.00072) = 0.63 W

For an ambient temperature, T_A, of 25°C, let’s compare T_J for the three packages:

T_J(SON) = 25°C + (58.3°C/W × 0.63 W) = 61.73°C

T_J(SOT223) = 25°C + (53.1°C/W × 0.63 W) = 58.45°C

T_J(SOT23) = 25°C + (205.9°C/W × 0.63 W) = 154.72°C

The junction temperatures for the first two packages are well within the recommended operating temperature range, but T_J(SOT23) exceeds the maximum temperature by a large margin. In fact, it’s dangerously close to the temperature (160°C) that activates the TPS732’s thermal-protection circuit.

A thermal-protection circuit is a standard feature on LDOs: When activated, it disables the output, protecting the LDO from overheating damage, and letting it cool. For the TPS732, when T_J cools to around 140°C, the thermal-protection circuit turns off, and the TPS732 resumes supplying current to the load.

If conditions don’t change, the part will heat up again and eventually reactivate the thermal protection. The LDO will continue to oscillate at some frequency that’s a function of the thermal-protection hysteresis, the power dissipation, and other variables. A calculation such as the one above is a standard part of the LDO design and would normally force a switch to a more thermally efficient package.

What other steps can the designer take to reduce the temperature rise and avoid thermal shutdown? Decreasing the thermal resistance between the LDO and the PCB is another sound strategy. If the LDO has a thermal pad, it should be soldered to the ground plane or attached to a heatsink. Much of the heat leaves the LDO via the pins, so increasing the size of the input, output, and ground planes will also decrease the thermal resistance. Most datasheets for Texas Instruments’ LDOs contain a detailed thermal analysis and layout recommendations.

Finally, you can reduce the value of P_D by adding a power resistor R_P in series with the LDO input. The resistor reduces the input voltage at the LDO input pin, and therefore the power that must be dissipated across the power transistor. For an input current I_IN, the voltage seen at the LDO input pin is reduced by (V_IN– I_IN× R_P).  

Two cautions:

• The resistor must be able to dissipate the power P_RP generated by the worst-case current (P_RP = I_IN(max)²× R_P)
• The voltage drop across R_P when I_IN(max) flows through it must leave the LDO input above the minimum needed for regulation: i.e., V_IN> V_OUT + V_DO.

When It All Goes Wrong: Overload Protection Circuits

The design techniques discussed above are necessary, but not sufficient. As we’ve seen in the previous section, the temperature of an LDO depends on P_D, which in turn depends heavily on the current I_OUT supplied to the load. If the load demands more current than the LDO is designed to supply, or during an abnormal condition such as a shorted load, the LDO also contains internal circuitry to limit the current to a predefined value and prevent catastrophe. 

4. The LDO “brick-wall” current limit shuts off the LDO output when the load current exceeds the limit value. (Source: TI blog: “LDO basics: Current limit,” Fig. 1)

Figure 4 shows the standard LDO current-limiting circuit. It works by measuring the output current, scaling it down, and comparing the scaled current to an internal reference current I_REF that represents the maximum allowed value. If the scaled output current exceeds I_REF, it triggers the comparator, which shuts off the output, and the voltage drops to zero. This type of circuit is commonly referred to as a “brick-wall” current limit.

Another strategy is to limit the maximum current the LDO can supply to a fixed value I_LIMIT independent of output voltage. V_OUT isn’t regulated when the device is in current limit: V_OUT = I_LIMIT× R_LOAD. If the load is shorted, the power transistor must be large enough to dissipate (V_IN× I_LIMIT) indefinitely—or at least until the thermal protection circuit is activated.

5. Two varieties of current limit: brick-wall (a) and foldback (b). (Source: TI blog: “LDO basics: Current limit,” Figs. 2,3)

Foldback current limiting is a third strategy in which the goal is to limit the total power dissipation rather than the output current. This approach keeps the output transistor within its safe power-dissipation limit by reducing the output current limit linearly while V_OUT decreases and V_IN remains steady. The advantage of this approach is that the foldback current is less than I_LIMIT above, so the power transistor must dissipate much less power, reducing the risk of damage. Figure 5 compares the brick-wall and foldback waveforms.

 Sponsored Resources: 

• Linear Power for automated industrial systems
• How hot is your LDO?
• LDO basics: Current limit

This platform sets the stage for Allegro to expand its product offerings in speed, electrical current, and angle sensing applications. Allegro's AxMR™ technology was designed to withstand the rigorous automotive under hood environment. AxMR™ technology is thermally stable to greater than 150°C, even in the presence of large magnetic fields.

Guerrilla RF, a startup that supplies low-noise amplifiers for cellular base stations and small cells, said on Wednesday that it had raised $3.8 million in financing. The financial refuel comes with more than 50 products based its massive microwave integrated circuits shipping in production volumes.

“This year is off to a very fast start for us,” said Ryan Pratt, co-founder and chief executive of Guerrilla RF, in a statement. “With multiple customer production ramps underway, it was clear we needed additional working capital.” He added: “Based on the revenue growth we see, we believe we’re in striking distance of profitability for the first time.”

The Greensboro, North Carolina-based company, which was founded after Pratt stepped down as an engineering director for Skyworks Solutions, has raised $11.6 million over the last five years. The company's investors include Bill Pratt, one of the founders of RF Micro Devices, which merged with Triquint Semiconductor to form Qorvo.

Back on September 1^st, 2010, I wrote Bob Pease asking about the early history of voltage references. An amazing fount of knowledge, Pease replied to my questions saying, “These are tough questions, and almost none of them are easy to answer. I could put in 1, 2, 4 hours trying to answer them. Let's see how far I get in 5 minutes.”

A Glowing Reference

My first question was what engineers used before bandgap voltage references. Pease replied:

“We used neon glow tubes such as the OA-2, OB-2, and the 85A2(Fig. 1), as used in Bruce Seddon'sR-300. Many semi-precision instruments in the vacuum tube era used those. I built a 4701-type V-to-F converter in 1995, using the OB2, and it wasn't bad at all, using all 1941 parts. I could have made a darned good DVM (digital voltmeter) back in 1941, ignoring the fact that I was only 1 year old, and I would have had to teach my mother how to do the soldering. That's the way it is, with time machines.”

1. The 85A2 neon voltage-reference tube had a metal cover to shield the tube from ambient light. Light striking the tube would lower the regulation voltage by ionizing some of the gas in the tube. (Courtesy of Mark Hippenstiel)

I note the 85A2 neon tube had a metal cover. That’s because incident light will change the trigger voltage. I see this with old plug strips with neon indicators. I have one where the indicator stays off in the dark, but when I turn on the room lights, the indicator starts flickering. The incident light ionizes some of the gas in the tube, so it successfully avalanches and glows despite being worn out.

This type of multiphysics reaction, as the people at Comsol might call it, affects a lot of systems. Years ago, Comsol showed me how they can simulate a transformer not just for its magnetics, but for how the self-heating of the core would change the magnetic properties and, hence, the electrical simulation. Another example they gave was calculating the dynamic flow through an aorta when the pressure would change the size of the artery.

Other systems that have multiphysics considerations are silicon oscillators, which will start up faster than a quartz crystal. Quartz crystals are also affected by shock, as they are a mechanical system as well as an electrical one.

Keen on Zeners

I went on to ask Pease what kinds of early references were there besides bandgaps. He noted, “Zener diodes and temperature-compensated Zeners such as the 1N821, 1N823, 1N825 families, and dozens of others of selected Zeners. These ran on 7.5 mA; the 1N4571 ran on 1/2 mA. These came along before silicon transistors. Standard cells have been used before that. Bruce Seddon in the R300 datasheet recommended that you could use a stack of 65 1.34-volt mercury cells, to replace the 85A2 for better long-term stability, but you can't buy them anymore.”

2. The temperature coefficient (TC) of a Zener diode depends on the breakdown voltage of the particular part. Below 5.6 V, the coefficient is negative, as dictated by the Zener effect. Above 5.6 V, the avalanche effect dominates and the TC is positive. (Courtesy of Wikimedia)

Zener diodes are interesting in that they can have a positive or a negative temperature coefficient (TC) depending on the breakdown voltage of the part (Fig. 2). Zener diodes with a breakdown voltage above 5.6 V have a positive TC. The breakdown voltage goes up with increasing temperature. Zeners with a breakdown voltage below 5.6 have a negative TC; the breakdown voltage falls at higher temperature. A 5.6-V Zener diode has negligible coefficient.

Like neon glow tubes, hot transformers, and quartz crystals, there’s a multiphysics factor. Diodes with breakdowns below 5.6 V are dominated by the Zener effect. Parts with breakdown voltages above 5.6 V are dominated by the avalanche effect.

3. An old Delco training manual shows the circuit of an early GM electronic voltage regulator. Zener diode D2 is in series with an emitter-base junction of a PNP transistor TR2. The negative temperature coefficient of the transistor offsets the positive coefficient of the Zener diode.

A trick we used back in the 1970s was to offset the TC of a Zener with an emitter base junction (Fig. 3). Automotive design is extremely cost-sensitive, so rather than use two 5.6-V Zeners to get close to the 13.75-V ideal battery charging voltage, Delco designers put an emitter-base junction in series with a higher-voltage single Zener. The positive TC of the Zener is offset by the negative TC of a transistor junction. Using the transistor as a part of the reference is cheaper than two diodes and a transistor used as a simple voltage-follower buffer.

Beginning of the Bandgaps

Next I asked when did Bob Widlar invent bandgap references. Pease expounded, “About 1968, when he brought out the LM109. That was well before the LM113, which was about 1971. He borrowed the idea from Dave Hilbiber of Fairchild from his old ISSCC (International Solid-State Circuits Conference) paper. Widlar took the (opposing) stacks of 11 and 10 NPN transistor Vbe's (base-emitter voltages), running on 10 volts, and folded them to work on 1.5 volts.”

I should note that Bob Dobkin, CTO of Linear Technology, part of Analog Devices, is on the patent application with Widlar. It was sensible to patent the idea. Back in the 1970s, it was pretty easy to cut open the metal can of an op amp or voltage reference and reverse-engineer the circuitry. Former Maxim application engineer Eric Schlaepfer, now at Alphabet, does this for fun. I was impressed when he did this for logic ICs. I was astounded when he started reverse-engineering 555 timer chips, 741 operational amplifiers, and 6502 microcontrollers.

Another innovator for bandgap references is Analog Devices’ Paul Brokaw, who wrote a great pamphlet about bandgaps when he was with Integrated Device Technology. One of his improvements was to add an op amp to the bandgap circuit, to improve stability and load regulation.

I have a signed copy of Pease posing as the “Czar of Bandgaps” (Fig. 4). I got it at a seminar in October of 1999. I asked him how he came to be called that. Pease replied, “Very simple - because about 40 to 60% of the bandgap IC's that National Semiconductor brought out in the 1980's, had old dumb errors, and I wanted to make sure we stopped making old dumb errors, and to make only a minimum of new dumb errors. So I declared myself the Czar of Bandgaps. to help minimize such errors, old and new. You can tell people where to find the photo of The Czar with garlands of LM117's (rejects). Many such errors are related to layout and can be caught at a good beer-check.”

4. National Semiconductor had fun with Bob Pease’s persona as the Czar of Bandgaps. They made him a great costume he could use in seminars and advertisements. This signed poster overlooks my lab bench and keeps me diligent.

I then asked Pease if anybody makes Zener references anymore. He noted, “I guess so. I ain't looked much, recently. LTC and NSC still make LM129's and LM329's. I'm sure you can still buy 1N823's and 1N825's" When I asked if he had any other comments about references, he said, “I wrote a whole article in some darned encyclopedia, with the whole history. Hundreds of years. But this may not be on the 'net. That was ~ 14 years ago.”

Back then I did not know of Linear Technology’s LTZ1000, one of the best voltage regulators you can buy at any price. Its back to the physics thing. The LTZ1000 uses a buried Zener. This means the semiconductor junction is below the surface of the IC die. This makes for less electrical noise since there are fewer impurities and crystal defects below the surface. This is the same reason JFET (junction field effect) transistors and op amps have less noise than many bipolar or CMOS parts.

I mentioned to Pease how I loved his article in Electronic Design about using multiple references to reduce noise. He noted, “Yeah, the art of paralleling and averaging is obscure, but not unknown. Of course, you have to start with a population of no drifters, and very few noisy ones... or else, sort out the bad ones, and keep the good ones. This is fairly labor-intensive.” Pease then signed off with his classic, “Gotta run. Beast [sic] regards. / rap”

Drifting Off

Drift, the slow inevitable changes in a reference’s output, was of special interest to Pease. The physics of TC are pretty well understood, but drift remains a mystery. In addition to Zener and bandgap references, the Intersil division of Renesas has a reference based on floating-gate technology. This is the same process as is used in flash memory. If you raise the voltage across a thin oxide layer, electronics will tunnel into the otherwise unconnected gate of a MOSFET (metal oxide field-effect transistor). This charges the gate up and sets a conductance of the FET.

When I learned of this type of reference, I asked Barry Harvey, then at Intersil, if the charged voltage would bleed off over time and at high temperatures. He assured me it was a matter of attoamperes, even at higher temperatures over decades. He also reminded me that the size of the gate was an IC designer’s prerogative. A large gate means a lot of charge that will persist over time and drift less. The downside is a large gate takes a lot of die area, but it’s thankfully only a small part of the total die cost. These types of references have a unique combination of noise and power consumption that may be ideal for your system.

Besides noise and drift, you have to understand there is an initial accuracy spec for references. Many system designers feel that drift and TC is what is important, since any digital system can calibrate out any initial accuracy deviation. Years ago, it was common for low-drift references to have an initial accuracy as bad as 5%. This got much tighter with the advent of lithium-ion battery charging. Because you have to control the charging voltage to 1% or better, a slew of references now have sub-1% accuracy.

Don’t forget about that startup time lesson from the quartz crystal. If you’re doing power management in your system, you may have to leave the reference powered, or wait a suitable time until the initial turn-on drift settles down.

Like much of analog, there’s much to know about references—parts that have only two or three pins. Keep digging and be sure to let the great application engineers at the manufacturers help you understand all of the tradeoffs and tricks. Bob Pease is no longer with us, but his passion to understand and explain is carried on by those engineers.

Well-designed electronic systems only use as much power as they need to for each state of operation. While this is crucial for battery-powered systems, AC-powered systems also benefit from minimizing power, since that reduces heat dissipation, maximizes the product lifetime, and conserves electricity.

Low-power modes work best when they seamlessly transition to a higher power mode without the user taking separate action. This full automation will be paramount to smart systems of the future. When the power mode can change based on some mechanical movement occurring, Hall effect sensors are often a suitable technology to be used.

Hall Effect Sensors

Semiconductor integrated circuits (ICs) with embedded Hall effect sensing elements are used all over the world in everyday products for measuring position. These magnetic sensor devices are used in personal electronics, industrial systems, medical devices, automobiles, aircraft, and spacecraft. Although there are other magnetic sensing technologies, Hall effect continues to be the most prevalent due to its unique set of advantages:

• Inexpensiveness: ICs that incorporate Hall effect elements are mass produced with standard CMOS processing flows.
• High reliability: being solid-state sensors that contactlessly measure magnetic fields, devices can operate for decades.
• Simplicity: while the inside of an IC incorporates thousands of complex circuits, the outside of most devices only has 3 pins. The output pin is a simple indicator of the proximity to a magnet, and standard microcontrollers can directly read it.
• Distance sensing: magnetic fields travel a distance and pass through most substances undisturbed. This allows sensors to be buried under enclosures where they are shielded from the environment and invisible to the user.

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The unique features of the DRV5000 family of Hall effect sensors, combined with the ultra-low power capabilities of the MSP430TM MCU, enable for the first time, smart physical presence-detection systems such as door or window security, and e-meters to remain directly powered from a single low cost CR2032 coin cell for over two decades of continuous operation.

Compared to an always-active sensor system, an intelligent Hall-effect sensor system can be designed with its current consumption reduced from what are typically milliamps to less than a microamp — a reduction of over 1000x. This is achieved by combining the fast start-up time and wide-range operating voltage of the DRV5000 Hall sensor with the ultra-low-power capabilities of the MSP430 MCU. Such a sensing system with extremely low power consumption enables physical presencedetection applications, such as door or window security systems, and e-meters to be directly powered from a single low-cost CR2032 coin cell for over two decades of continuous operation.

The techniques described in this paper that enable ultra-low power (ULP) sensing include:

• Operating the system in ultra-low power standby mode as the normal mode
• Duty-cycling the sensor
• Exercising power-aware firmware

The simplified example circuit in Figure 1 shows a very low-cost system that does not require any external calibration. The complete sub-microamp, intelligent Hall-sensing system detects the presence of a magnet and is powered directly from a CR2032 battery.

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Each time you climb into your car, you enter a world of magnetic sensing, with tiny sensors reporting whether the doors are closed and seat belts buckled properly. Sensors also automatically guide seats into place, help open windows, turn steering column switches on and off, and track how you turn the steering wheel or press the accelerator.

Several magnetic sensors are located, under the hood, detecting speed and position in the car’s engine, transmission and electric motors. Other sensors monitor fluid levels, track body and wheel position, perform a number of safety checks, and sense currents for a variety of electrical feedback and measurement functions.

A new automobile today may have as many as 70 magnetic sensors to enhance operation, safety and convenience. More are being added all the time due to technology advancements that deliver greater accuracy and reliability in smaller sizes with lower cost.

The same trend can be observed in the electronic and electromechanical world in general. In addition to widespread use in vehicles, magnetic sensors find application in equipment including industrial motors and robots, medical systems, office machines, home appliances, and even handheld tablets and cellphones. Like so many other semiconductor devices that make the modern world go round, magnetic sensors are invisible to end users but indispensable for many of the functions that we have come to take for granted. Figure 1 lists some of these uses.

What enables the ubiquity of magnetic sensors today is their small size and affordability. Semiconductor manufacturers have applied advanced production techniques to enable analog integrated circuit (IC) products that include sensors, bringing the advantages of miniaturization to what were once space-consuming devices. At the same time, the cost of the permanent magnets integrated within some magnetic sensors has dropped, helping push the trend toward affordability that comes with advanced manufacturing, as well as increasing functionality. The need for improved reliability, safety and accuracy has motivated end product developers to take advantage of the inexpensive magnetic sensors that are now available on chips along with other circuitry. As a result, applications are mushrooming, with a market estimated to pass $2 billion and 20 billion units in the next five years, with the majority in the automotive segment.

Texas Instruments (TI) devotes significant development effort to magnetic sensing. TI’s portfolio of magnetic sensors gives equipment manufacturers an array of options for their varied application needs. The company’s advanced analog process technology makes possible the on-chip integration of complete sensing solutions that include both the sensing element and other circuitry. TI process innovation enabled the industry’s first integrated fluxgate magnetic sensors for extremely high-precision measurement. For the growing world of magnetic sensing, the company’s technology continues to drive the development of solutions for an ever-expanding range of end uses.

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At the time of writing, some colleagues are working on a demonstration for the big “Embedded World” show in Germany. They’ve plumbed in some natty speech-recognition technology into our super new dual-core PSoC. The buzz-phrase for this is “wake phrase recognition,” and all you up-to-date people out there have no doubt already used it when you intone “OK, Google” or “Hey, Siri” or “Dang, Alexa, I did not order paper towels.” The difference is that for personal-scale embedded devices like watches, rings, and earbuds without a permanent connection to “The Cloud,” the processing must be done locally and at super-low mean power.

I only mention it because this “wake phrase recognition” will be essential for the Engineer’s Smart Watch that I feel I must crowdfund rather urgently. It’s a special Smart Watch to be worn by any engineer who gets involved in electronic circuit or system design. Its unique feature is that it will deliver a painful electric shock to the wearer upon detecting a preset wake phrase of my choice. And the very first phrase that will go on its list is:

“voltage at”

as in:

“I measured the voltage at the junction of R5 and C3.”

Accepting this phrase at face value is a symptom of a dread disease. I refer to this ailment as “Kirchhoff’s Madness,” and its main carrier is an imaginary beast that I call the One-Leaded Voltmeter. I say imaginary, because none of the meters I’ve ever seen for sale here in the real world have just one lead; two leads seem to be very much the order of the day.

The problem has become far more prevalent in this age of circuit simulation, where map is assumed identical to territory. Here’s one example I found in the wild after a quick search:

Now, I can live with this hypothetical one-terminal flourish when the currency of your simulation is a signal of unspecified (and irrelevant) physical form. Here’s a less harmful version, also found on the web:

In this type of diagram, the signals flowing along the connecting lines are just dimensionless carriers of information in a system-level simulation. No electrons are harmed or excited in the acquiring of that information.

What’s the Point?

What, then, is the problem that is getting me all hot under the collar? It’s this: Voltage is something that exists between two points, not at one point. You may recall from the physics of your youth that the potential between two points is the definite integral of the electric field along a path between those points (any path). The definite integral is required in order to eliminate the constant of integration. Trying to “measure” a “voltage” with a one-leaded meter is like trying to calculate a numeric value from an indefinite integral—you haven’t defined a path over which to do the calculations.

Of course, when pressed, the sufferer of Kirchhoff’s Madness will protest that the other lead of the voltmeter has been connected to (and here they often make a kind of reverential gesture) GND, by some higher authority. The existence of GND is axiomatic to their worldview, and all measurements of voltage require only the one true lead.

Now, there are circumstances where the temptation to take the two leads of a real voltmeter in two hands and go probing between points could be harmful. Experienced engineers can tell you that while low-voltage electronic systems can be designed with one hand behind your back, high-voltage systems are hazardous enough that you must work on them with one hand behind your back. In that case, you need to carefully attach “the other lead” of your voltmeter to one carefully selected reference point and leave it there while you go exploring with the other lead, monomanually.

Back to the watch and the punishment shock. The moral of today’s tale is this: Always make it clear where both leads of your voltmeter are connected, when you declare that you have measured a voltage. If there’s an unambiguous place where you feel comfortable in parking “the other lead,” write down what that place is, and attempt to show it on the circuit diagram of your design. This will, of course, rapidly illustrate that almost every modern electronic circuit diagram fails explicitly to define this one critical node.

A useful tip is to take a photograph of the setup in front of you, so that everyone can see where that other lead went. This can help the more experienced members of your team figure out that the reason your measurements or ‘scope traces suck is that this was a bad place to connect that lead. #NotKidding.

To and Flow

Another common symptom of Kirchhoff’s Madness is the belief that current flowing down a wire is akin to water flowing in a pipe, as delivered to you by the water company. You do what you want with this water, but it never gets back to the water company, instead just soaking into your lawn or draining to a sewer somewhere.

When (mis)applied to electronic circuits, this thinking commonly leads the incautious into trouble. The temptation is to believe that currents that flow to—look, there it is again—GND, somehow disappear, like the spider that a parent brushes off the bed with a casual “it’s gone now.” Nope. That current is still flowing, in a closed loop that will eventually get it right back to where you first encountered it—after it has had some interesting adventures, causing trouble in places you weren’t looking. A bit like that spider.

Lest you should feel that I’m lecturing you from the ivory tower vantage point of “well that never happens to me,” let me confess one manifestation (out of several) of Grrrr-ound that catches me out to this day. The only way to really appreciate a mistake is to make it yourself (like cake or a good cocktail, I suppose). This one’s due to the ground lead on the scope, which, under most circumstances, should be robustly attached to your choice of GND node on the board.

But what if that’s the only way that “GND” can actually get to your board, because of some power-supply wiring mess-up?  If you never test the board without the scope ground (or BNC lead ground, same effect) then you’ll never see it not work. But the careless measurer with the one-leaded scope will detect the defect immediately…

There may well be more tales from “The Chronicles of GND” in future Front End pieces. For now, can you think of a time when the “voltage at” shock would have worked as a wake-up call to ensure you connected your voltage measuring device correctly? Let me know!

The past week has seen two notable announcements regarding investments in chip-design capability from major players in the internet services realm. Alibaba Group and Facebook are taking different approaches to establishing their semiconductor strategies. However, both support the growing trend for major downstream companies to establish direct control and/or ownership of semiconductor capability as part of their product development strategies.

For many years, it’s been commonplace for electronics OEMs to possess their own semiconductor design and even manufacturing capabilities. Companies often find it necessary to dedicate funds to semiconductor research in their R&D efforts to develop technologies critical to their products. 

For instance, companies in the defense industry often have highly integrated design operations for their products. Meanwhile other players such as printer and storage companies have directly controlled key semiconductor components such as read/write controllers or print heads in their designs. 

Swimming Downstream

Firms like Microsoft, Google, and Amazon are among major players in the downstream software and internet services world to invest directly in semiconductor design capability. The chip investments by Alibaba Group and Facebook illustrate growing momentum for leading internet and software services companies to integrate semiconductor design more directly into their overall product strategies. 

Alibaba Group is jumpstarting its efforts through an acquisition while Facebook is pursuing organic development in its semiconductor efforts. Alibaba Group announced its acquisition of C-Sky Microsystems, a designer of China’s homegrown 32-bit embedded CPU processing core. In a separate approach, it was revealed that Facebook is building a team to design its own semiconductors. According to a job listing on its corporate website, the social media giant is seeking to hire a manager to build an “end-to-end SoC/ASIC, firmware, and driver development organization.”

While there are common motivating factors, such as a desire to more tightly couple software, hardware, and semiconductor design efforts and control costs and schedules, unique elements are driving each company’s actions, too. Facebook’s posting indicates it wants to develop chips as part of its artificial-intelligence developments. Its job post seeks “expertise to build custom solutions targeted at multiple verticals including AI/ML.”

Areas where Facebook could deploy its own semiconductor designs are in development of its own hardware devices, artificial-intelligence software, and servers in its data centers. Examples of the types of hardware coming from Facebook are the Oculus Go, a $200 standalone virtual-reality headset, and a variety of upcoming smart speakers.

Alibaba Group’s investment in semiconductors is well-aligned with the larger initiative championed by the Chinese government to establish a self-sufficient semiconductor industry.  While Arm and MIPS currently dominate the licensing space for embedded processor cores, C-Sky has designed a 32-bit high-performance/low-power processor architecture that it has successfully licensed to 70 companies in China so far. C-Sky boasts a series of embedded cores, SoC platforms, software tools, and middleware.  Through its previous collaboration, C-Sky has already designed CPU cores that are aligned with Alibaba’s proprietary embedded OS, Yun OS IoT.  It will likely play a key role in ongoing development of IoT edge devices connected to the cloud. 

Counterpoint: Successful Semiconductor Partnerships

The trend to direct semiconductor control by downstream players doesn’t mean that merchant-market chip suppliers won’t continue to be critical partners in the product designs of these companies.

During the same week as the Facebook and Alibaba Group announcements, MediaTek announced it’s working with Microsoft to deliver the first Azure Sphere chip, the MT362, this year. Azure Sphere is designed for highly secured, MCU-powered connected devices at a price that the companies expect will make enterprise-class security affordable for an array of cloud-connected gadgets.

Working together, the two companies have developed solution that includes a Wi-Fi-connected controller built around a processor that runs Azure Sphere’s IoT operating system. This system will provide support for Microsoft’s latest security protocols.

Block diagram of MediaTek's MT3620 for the Azure Sphere.

MediaTek is the reference chip partner for Microsoft’s Azure Sphere Partner Program. The MediaTek MT3620 is the first Azure Sphere-certified chip, which the companies expect will be the foundation for a new generation of secure intelligent edge devices and solutions. Microsoft plans to build a new ecosystem around its Azure Sphere platform, consisting of silicon vendors, ODMs, and OEMs from a broad range of industries.

These recent announcements by major players in the technology space provide evidence that the world of semiconductors is becoming more complex as organic development, acquisitions, and partnerships combine to form a wide variety of semiconductor product-development strategies.

There’s no doubt that as electrical engineers, we’re in a complex and fast-moving profession. Each market segment—automotive, consumer, aerospace, industrial, etc.—has its own set of unique product requirements and applicable standards (Fig. 1). At the same time, there’s a push across all segments to do more with less: higher performance, smaller packages, and lower power consumption, for example. 

1. Aerospace and defense applications must maintain a high level of performance under extreme conditions. (Source: “TI Components for Space, Avionics and Defense” PDF)

It’s tough to keep up with the latest developments in a particular field. Some estimates place the half-life of an engineering degree at between 2.5 and 5 years, and the time needed to stay current at up to 20 hours per week. The result? Working engineers must continuously brush up on their knowledge if they’re to remain state-of-the-art.

Engineering Resources from Texas Instruments

Not that we want to blow our own trumpet (well, okay…), but Texas Instruments (TI) is at the forefront of industry efforts to help working engineers upgrade and update their skills.  That commitment stretches back decades to legendary collections of application notes from companies such as Burr-Brown, Unitrode, and National Semiconductor. All three are now part of TI, and much of that material is still available online.

Download this article in PDF format.

There’s no doubt that as electrical engineers, we’re in a complex and fast-moving profession. Each market segment—automotive, consumer, aerospace, industrial, etc.—has its own set of unique product requirements and applicable standards (Fig. 1). At the same time, there’s a push across all segments to do more with less: higher performance, smaller packages, and lower power consumption, for example. 

1. Aerospace and defense applications must maintain a high level of performance under extreme conditions. (Source: “TI Components for Space, Avionics and Defense” PDF)

It’s tough to keep up with the latest developments in a particular field. Some estimates place the half-life of an engineering degree at between 2.5 and 5 years, and the time needed to stay current at up to 20 hours per week. The result? Working engineers must continuously brush up on their knowledge if they’re to remain state-of-the-art.

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• Maximize your Data Converter Performance, while Minimizing Performance Degradation
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Engineering Resources from Texas Instruments

Not that we want to blow our own trumpet (well, okay…), but Texas Instruments (TI) is at the forefront of industry efforts to help working engineers upgrade and update their skills.  That commitment stretches back decades to legendary collections of application notes from companies such as Burr-Brown, Unitrode, and National Semiconductor. All three are now part of TI, and much of that material is still available online.

On the TI website, you’ll find a huge selection of technical resources. There’s product information, of course, but also:

• Design tools and software such as those in the WEBENCH Design Center
• Reference designs that go beyond the datasheet to show how to solve common design problems, how to use a device in a subsystem, or how a device performs in a specific application
• Online video training and tutorials
• The E2E support forums and blogs
• Design seminars
• Technical articles, white papers, and application notes
• Products and services from the TI Design Network

The material covers all of Texas Instruments’ product lines, so you’re encouraged to visit and explore. 

Space, Avionics, and Defense Resources for Engineers

As an example of the available resources, let’s take a closer look at the Space, Avionics, and Defense market. This industry includes aircraft systems for engine control and flight control; ruggedized communications for defense and avionics; smart munitions; and a broad range of space-bound applications.

Products in this segment must operate over years, if not decades, in an extremely harsh environment. In addition, space products must survive exposure to high levels of radiation, so they must meet specialized testing and qualification requirements.

TI Space Products comprise the largest selection of radiation-hardened and assured products for space flight. TI has a proven legacy of over 45 years in the space market, offering a full signal-chain solution for both domestic and international programs.

The product portfolio includes rad-hard point-of-load (POL) power solutions, high-speed serializer-deserializer (SerDes) devices, and some of the world’s highest-performance data converters. The portfolio features components that comply with MIL-PRF-38535 QML Class V and Radiation Hardness Assured (RHA) standards. These devices are typically supported with total ionizing dose (TID) and single event effects (SEE) test reports that detail potential degradation in a space environment.

For earthbound aerospace and defense applications, TI’s Enhanced Products (EP) portfolio includes more than 750 commercial off-the-shelf (COTS) products that meet avionics, defense, and industrial standards for operating in environments where high quality and long service life are a requirement. Though these are products designed for harsh environments, more than 90% fall under the U.S. Department of Commerce EAR99 classification and can be sold worldwide with no restrictions.

The TI website has multiple resources to help educate yourself about the products and their application.

If your product is heading toward blast off, check out the Final Frontier section of the site. You might start by reading the Analog Wire blog entitled, “7 things to know about spacecraft subsystems before your next trip to Mars.” Do you know them all?

At a more detailed level, there are numerous resources on the quality and reliability specifications pertaining to space-bound products, such as QML conformance and the QML Test Flow Matrix, as well a series of videos on TI’s Rad Hard processes and products.

2. Listed are the topics covered in the Aerospace & Defense Training Series. (Source: TI Training: “Aerospace & Defense Training Series”)

There are many resources closer to ground, too. The Aerospace & Defense Training Series consists of 17 videos, lasting almost five hours, that cover specific products and applications (Fig. 2). The series contains six sections and a webinar. Several sections require you to login with your MyTI username and password. If you don’t have one, register free at my.TI.com.

The webinar on op-amp technology is scheduled for May 17, 2018. The seminar will cover the different op-amp process technologies, key specifications, and how to choose the correct op amp for your application. Registration is required, and attendees will have the opportunity to participate in a live Q & A session.

The Space, Avionics, and Defense home page has many more resources on each of the market segments: aircraft engine control, communications, sensors, imaging, smart munitions, radios, sonar, inertial navigation, and more.

Deep Dive Example: High-Speed Data Conversion

Let's drill down further and employ some of the resources mentioned to help with a task that's common in both defense and test-and-measurement applications: high-speed data conversion. The analog front end (AFE) in such a design requires analog-to-digital and digital-to-analog converters (ADCs and DACs) capable of handling gigahertz RF signals. In aerospace and defense, applications include radars, tactical radios, and electronic-warfare (EW) systems. Test-and-measurement designers use high-speed AFEs in digital storage oscilloscopes (DSOs) and 5G wireless test systems.

3. Here’s a generic block diagram of a high-speed AFE for radar, military radio, and similar applications.

The signal-chain block diagram (Fig. 3) for the AFE looks similar for each application. On the input side, the incoming RF signal is buffered, and then passes through a low-lass filter and an RF attenuator. The balun converts the single-ended signal to a differential format that feeds a high-performance differential amplifier. Finally, a high-speed ADC samples the RF signal directly and sends it to the digital core (FPGA or ASIC), which performs digital signal processing on the data stream to extract the signal of interest. The output side is basically a mirror image of the input circuit.

Resources for RF Analog Front-End Design

What resources are available that shed light on the requirements for this application?

Introductory Topics: If you’re new to the field, start with “High Speed Signal Chain University.” This video training series covers high-speed RF sampling data converters, high-speed amplifiers, the JESD204B standard, and related topics.

In part of Signal Chain University, click on “Introduction to the RF Sampling Architecture”, where Russell Hoppenstein discusses the changes to the traditional superheterodyne receiver architecture (Fig. 4) that are required by these high-speed applications.

4. New RF applications replace the traditional superhet architecture and its quadrature demodulator (a) with a high-speed ADC that converts the RF signal directly (b). (Source: TI Training: “Introduction to the RF Sampling Architecture”)

Other videos in the RF sampling series discuss the theory behind RF sampling, and issues related to managing the gigabit-per-second data stream. If you prefer the written word, consult the resources at the RF Sampling Learning Center.

Serial Interface: Both the ADC and the DAC in the block diagram in Fig. 3 communicate with the logic core over a JESD204B serial interface. The JESD204B is a new JEDEC standard that defines the communication between high-speed ADCs and DACs and a digital device such as an FPGA or ASIC. The “B” revision accommodates a data rate up to 12.5 Gb/s. Compared to previous solutions based on low-voltage differential-signaling (LVDS) technology, the JESD204B implementation allows for a reduced package size, a simpler PCB layout, and simplified interface timing, but consumes comparable power for the same throughput.

The TI site has many resources on JESD204B. This presentation reviews the current “B” specification, compares it to earlier versions, and then discusses several layers and their implementation.

High Speed Signal Chain University offers a JESD204B Video Blog Series that discusses the standard as it applies to high-speed data converters. And, of course, there are numerous white papers, application notes, and reference designs that discuss different facets of the standard and practical applications. For example:

• White Paper: Ready To Make The Jump To JESD204B?
• TI Analog Application Journal 1Q 2014: When is the JESD204B interface the right choice?
• “Analog Wire” blog series on JESD204B from multiple TI engineers—just go here.
• TI Design TIDA-001023: High-Channel-Count JESD204B Clock Generation Reference Design for RADAR and 5G Wireless Testers
• Application Report SNAU222: JESD204B Multi-Device Synchronization Using LMK0461x

ADC and DAC Selection: The ADC and DAC part numbers vary with the application. The table shows the recommended parts for several defense-related applications.

Why are these devices recommended for the target applications? Philip Pratt reveals all in video 3.4 of the Aerospace & Defense Training Series, entitled “Data Converter Solutions for Defense Systems.” Again, fans of the written word can peruse the related course material.

Last, but not least, you can browse the product overview pages. These include selection guides, reference designs, information on other applications, links to TI training, and the product datasheets.

Here’s the overview page for high-speed ADCs, including the RF-sampling devices in the table. And here’s the high-speed DAC overview page. 

Reference Designs

As you research the application, download datasheets, and visit sections related to your design, the site will offer suggestions based on your activity. Figure 5 shows some of the suggested reference designs for a high-speed data-acquisition system for radar, test and measurement, and wireless applications.

5. Suggested reference designs for a high-speed data-acquisition system for defense and test-and-measurement applications. (Source: TI Reference Designs)

The reference designs cover various aspects of the AFE design: the RF sampling front end, DSP, and multichannel clock management. A reference design typically includes:

• Schematic
• Design guide with verified test and simulation data
• Design package: assembly drawing, CAD files, Gerber file, layer plots, etc.
• Complete bill of materials (BOM)

Examine the reference designs in Fig. 5 to find out more.

Conclusion

Although this article covered the resources to help with one design task in one market segment, you can find a similar depth of coverage across the many market segments supported by TI. For example, in consumer electronics, check out the material on the new USB Type-C standard and the USB Power Delivery (USB PD) specification; in automotive, see the presentations on EV and hybrid architectures.    

The goal is not only to provide the right products, it’s to deliver the tools, education, and knowledge that’s needed to stay current in a world where, if you pardon the cliché, the only constant is change.

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If you’re designing circuits and equipment that require electrical/electronic isolation, it may be time to consider electronic isolation via capacitance. Of the methods available, capacitive isolation provides outstanding advantages over magnetic isolation by transformer or optoisolation with an LED and photodetector.

Such isolation is a common requirement in most industrial and medical applications. That’s where capacitive-isolation ICs, which have been developed and refined for implementation into these critical designs, step in.

Isolation Defined

Isolation is the process of blocking some signals and electrical connections while allowing others to occur. Known as galvanic isolation, this process prevents direct electrical contact between input and output, but allows for the transfer of signals. For instance, a typical isolator prevents dc or ac supply voltages from being passed on, yet at the same time permits data signals to pass.

A major function of isolators is to separate the common grounds of input-signal devices and the equipment receiving the signals. Using a single common ground almost always introduces ground loops and the attendant unwanted offset voltages.

Keeping high voltages as great as 10 kV from industrial equipment away from computers, sensitive equipment, and human operators is another function of isolators. In addition, isolators protect sensitive equipment from electrostatic discharge (ESD), electrical fast transients (EFTs), and other variations from electrical surges that are common in an industrial setting.

Such protection gives isolated equipment good electromagnetic compatibility (EMC) as required to meet selected certification standards. Capacitive-isolation ICs meet all of these requirements while supporting high-speed data rates and lower power consumption over other methods.

Common Isolation Methods

Figure 1 shows the three common isolation methods. The transformer in Fig. 1a is the most obvious—it uses two electrically isolated windings on a common magnetic core. Signals are passed by magnetic induction from primary winding to secondary winding. The isolation is excellent, but transformers have some downsides. They’re typically larger, heavier, and more expensive than other options. Though they do a good job of blocking dc, their frequency response can limit data rate unless special high-speed transformers (e.g., Ethernet) are used.

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If you’re designing circuits and equipment that require electrical/electronic isolation, it may be time to consider electronic isolation via capacitance. Of the methods available, capacitive isolation provides outstanding advantages over magnetic isolation by transformer or optoisolation with an LED and photodetector.

Such isolation is a common requirement in most industrial and medical applications. That’s where capacitive-isolation ICs, which have been developed and refined for implementation into these critical designs, step in.

 Sponsored Resources: 

• How To Simplify Isolated 24-V PLC Digital Input Module Designs
• How to Design Isolated Comparators for ±48V, 110V and 240V DC and AC Detection
• How To Improve Speed and Reliability of Isolated Digital Inputs in Motor Drives

Isolation Defined

Isolation is the process of blocking some signals and electrical connections while allowing others to occur. Known as galvanic isolation, this process prevents direct electrical contact between input and output, but allows for the transfer of signals. For instance, a typical isolator prevents dc or ac supply voltages from being passed on, yet at the same time permits data signals to pass.

A major function of isolators is to separate the common grounds of input-signal devices and the equipment receiving the signals. Using a single common ground almost always introduces ground loops and the attendant unwanted offset voltages.

Keeping high voltages as great as 10 kV from industrial equipment away from computers, sensitive equipment, and human operators is another function of isolators. In addition, isolators protect sensitive equipment from electrostatic discharge (ESD), electrical fast transients (EFTs), and other variations from electrical surges that are common in an industrial setting.

Such protection gives isolated equipment good electromagnetic compatibility (EMC) as required to meet selected certification standards. Capacitive-isolation ICs meet all of these requirements while supporting high-speed data rates and lower power consumption over other methods.

Common Isolation Methods

Figure 1 shows the three common isolation methods. The transformer in Fig. 1a is the most obvious—it uses two electrically isolated windings on a common magnetic core. Signals are passed by magnetic induction from primary winding to secondary winding. The isolation is excellent, but transformers have some downsides. They’re typically larger, heavier, and more expensive than other options. Though they do a good job of blocking dc, their frequency response can limit data rate unless special high-speed transformers (e.g., Ethernet) are used.

1. Common electrical isolation methods include inductive (a), optocoupler (b), and capacitive (c).

Optocoupler ICs have long been a popular isolation device (Fig. 1b). The signals to be passed are sent to an internal IR LED that switches off and on with the logic signal input to activate a phototransistor that turns off and on at the output. Isolation is excellent because of the insulated separation of the LED and photodetector.

The third option is capacitive (Fig. 1c). The signal path is via a capacitor with its insulating dielectric. A capacitive isolator readily blocks dc, but easily passes high-speed data signals and provides the ESD and transient protection indicated earlier.

Modern capacitive-isolated digital input receivers can also help simplify system design. When compared to optocouplers and other techniques, this new design approach results in advantages that include lower power dissipation, smaller boards and modules, simplified system design, and higher-speed operation.

How Capacitive Isolators Work

Figure 2 shows a simplified diagram of a capacitive isolator. The input is applied to two resistors: a sense resistor (R_SENSE) that establishes the amount of current drawn from the input sensor or device, and a threshold resistor (R_THR) that sets the input logic thresholds. A comparator with hysteresis shapes the input data signal for transmission to the output across a silicon-dioxide isolation barrier.

2. Shown is a functional block diagram of a capacitive isolator such as the TI ISO1211.

Since capacitive coupling blocks dc, how do logic signals get from input to output? The answer is modulation. Capacitive isolators implement a transmitter that uses on-off keying (OOK) modulation to pass the input data through the capacitive-isolation barrier. OOK is a variant of amplitude-shift keying (ASK). A binary 1 (typically +24 V) input turns on a high-frequency carrier that’s transmitted through the capacitor barrier. A binary 0 produces no carrier. The receiver circuitry demodulates the OOK signal envelope and reproduces the data by way of an output buffer stage.

Representative of the capacitive isolators available today are Texas Instruments’ ISO1211 and ISO1212. These devices have digital input receivers that can operate over a 9- to 300-V dc (24 to 60 V typical) or ac input range with external resistors setting the current and voltage limits. They’re compliant to IEC 61131-2 specifications for Types 1, 2, and 3 inputs, and don’t require a field-side power supply. The ISO1211 is a single channel device, while the ISO1212 has two channels.

These devices can operate with a dc supply voltage in the 2.25- to 5.5-V range with low power dissipation. Input voltage protection is inherent with reverse voltage protection (±60 V). The ICs offer an excellent alternative to optocouplers for programmable logic controllers (PLCs), motor controls, and other industrial equipment. They support clock rates up to 4 MHz where typical optocouplers are limited to rates of about 20 kHz. Typical propagation delay is 140 ns compared to about 20 µs for an optocoupler.

An evaluation module for the ISO1211 is available for experimentation (SLLU258A).

Design Examples Using Capacitive Isolators

Isolation modules are used in wide range of industrial applications. PLCs, motor drives, and CNC-controlled machines in factories, process control plants, and warehouses represent several examples. This equipment gets its inputs from switches, sensors, or dc and ac voltage sources.

3. A 24-V or other dc source is connected to the IN and SENSE pins of the TI ISO1211.

Isolators are used to eliminate the ground-loop problems associated with long lines connecting two sources and to provide high-voltage protection. One typical arrangement is shown in Figure 3. A switch, relay, or sensor connects 24 V dc to the isolator through a threshold resistor (R_THR) and a sense resistor (R_SENSE). The isolator output connects to a host microcontroller.

Some applications require monitoring remote dc or ac voltages from power supplies, battery monitors, or some relay-controlled device. To monitor ac, a bridge rectifier like that in Figure 4 is used. The ac is rectified into dc and filtered with C_IN to minimize ripple to the SENSE input on the IC. Resistors limit the input voltage to the isolator. A dc source may be monitored via the capacitive isolator through a voltage divider to bring the input voltage below the upper limit of the isolator using the arrangement in Fig. 3.

4. To monitor an ac source, a bridge rectifier is used to condition the ac into a proportional dc. C_IN filters out the ripple.

Capacitive-isolated comparators offer a reliable and low-power alternative to optocouplers for ±48-, 110-, and 240-V dc and ac detection. Advantages of using isolated digital inputs include precise voltage thresholds, low input current draw, higher speed, lower failures in time (FITs) higher reliability, and operation up to 125°C.

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• How To Improve Speed and Reliability of Isolated Digital Inputs in Motor Drives

Bob Pease had a special love for voltage-to-frequency converter circuitry. I did a vamp on his writing in an article of my own. If you want to measure a voltage in a remote location, it might be easier to send a series of pulses over a long wire, rather than send the voltage itself. The pulses are more resistant to noise, and you can clean the noise out of the pulse signal easier than try to reject noise out of a pure analog voltage. V-to-F circuits also have great dynamic range and will work over several decades of frequency.

Measuring a pulse-train frequency just requires a microcontroller or logic circuit using a cheap accurate crystal. That can be more cost-effective than measuring a voltage, where you need an accurate reference chip.

Still, your system might have an accurate reference anyway. When I designed automotive diagnostic equipment at HP, I noted that, “You need a rock and a ref.” This was a jaunty shorthand to say you needed a quartz crystal oscillator for accurate time, and a good reference to measure voltage. With those two, you can derive current, and power, and most of whatever else you want to measure. In a way, using a V-to-F converter means you’re pushing the reference requirement to the sensing chip. If you can develop an accurate V-to-F circuit, then you don’t need a separate reference chip.

Saying the V-to-F has great dynamic range is a way of saying the output is very linear. It’s accurate over a wide range of input voltages and output frequencies. That’s a great deal, compared to the headaches of ensuring a sampled data system remains accurate no matter the range of the input voltage. Once you add attenuation and range amplifiers to a system, things get pretty difficult and complicated.

I tried to use analog switches to make an attenuator in that HP diagnostic equipment I worked on. I had all kinds of problems with stray capacitance and accuracy issues, so I called Jim Williams at Linear Technology, now part of Analog Devices. He asked what I heard when I changed the vertical range on my Tektronix scope. I said that I heard relays clicking. Williams said they used relays since doing a solid-state attenuator is nearly impossible for high frequencies and voltages. Using a V-to-F converter might save you all of these headaches.

The LM331 and V-to-F

1. The block diagram of the LM331 represents a relaxation oscillator that will change frequency with input voltage. The output is a pulse train, not a square wave; hence the one-shot. (Courtesy of Texas Instruments)

To understand V-to-F conversion, the LM331 datasheet is a great place to start. The applications section was written by Pease. Several application notes published by Texas Instruments were most likely also written by Pease. These notes are all linked to on the LM331 product page. The fundamental app note, “Versatile Monolithic V/Fs can Compute as Well as Convert With High Accuracy,” was released in 1980, at the same time as the part. The app note shows both a simple and detailed block diagram of the part (Figs. 1 and 2).

2. A more detailed block diagram of the LM331 reveals the R-S flip-flop, the internal bandgap voltage reference, and a current mirror. The bandgap output voltage is available on pin 2, which you also use to set the internal current mirror with a load resistor. (Courtesy of Texas Instruments)

You can also use the LM331 to make a frequency-to-voltage converter. The app note “Frequency-to-Voltage Converter Uses Sample-and-Hold to Improve Response and Ripple” shows how to remove the output ripple from the circuit while still keeping a speedy response time. You can think of it as a form of synchronous demodulation. It samples the output voltage at the same point in the ripple waveform. This note builds on the basic F-to-V app note, "V/F Converter ICs Handle Frequency-to-Voltage Needs."

Another vamp on the basic V-to-F converter is the current-to-frequency converter. There’s another nice app note for that, “Wide-Range Current-to-Frequency Converters.” The app note describes ways to build on the inherent wide dynamic range of the LM331 to make circuits that can work down to picoamperes of input current. While you have to add a lot of circuity to get that performance, it’s a testament to the versatility of the LM331.

3. The folks at Boldport make a nifty kit based on a Pease application circuit in the LM331 datasheet.

Going through the drawers of my home lab bench, I uncovered an LM331 voltage-to-frequency IC demo board given to me by the folks at Boldport(Fig. 3). Dr. Saar Drimer, the electronic artist behind Boldport, packages the board in a handy kit that you can order from his website. The kit has a Bob Pease quote “My favorite programming language is solder.” The kit is based on Figure 20 from the LM331 datasheet (Fig. 4).

4. This LM331 datasheet circuit is the basis for the Boldport kit. The kit substitutes a photocell for the photodiode, adds an output LED, and slows down the frequency so you can see the LED blink. (Courtesy of Texas Instruments)

Drimer added an output LED so that you can see the output frequency change. Another difference is that while Pease called out a phototransistor, Drimer’s kit uses a photo-conductive cell. Drimer also slowed down the frequency so that you can see the LED blink. Micah Scott has a video build of the kit:

As does Ladyada from Adafruit Industries:

The Art of the Science

Most of us have heard about STEM (science, technology, engineering, and math). I first heard the term “STEAM” from my pals at Evil Mad Scientist. The added “A” stands for “art.” Boldport is an advocate of the artistic side of engineering, as evidenced by the whimsical traces on the LM331 Pease PCB (printed circuit board). While the artistic aspect of circuits seems to be blooming, it’s nice to know that analog geniuses like Jim Williams used to make working sculptures out of electronic circuits decades ago. Williams’ best friend, Len Sherman of Maxim Integrated, even made up a book of various Williams creations  (Fig. 5).

5. Here’s one of Jim Williams’ working electronic art pieces from the book put together by his friend Len Sherman. It’s as if a Digi-Key truck hit an Alexander Calder sculpture. (Courtesy of Len Sherman)

We analog folks also know there’s art in the circuit theory and the mathematics that underlay the theory. It might be easier to buy a cheap microcontroller from Microchip, TI, or NXP than use an LM331. The thing is, the design and implementation of the LM331 is worth understanding just so you know the principles of analog design. While chips come and go, the principles of analog are timeless.

Integrated power modules offer advantages over less expensive, discrete regulators. This is especially true for designs with many power rails and where high current is involved. Power modules provide validated solutions that simplify overall system design by reducing component count and qualification. They are typically optimized for EMI and thermal performance.

Texas Instruments’TPSM82480 DC/DC module(see figure) integrates power metal-oxide semiconductor field-effect transistors (MOSFETs) and shielded inductors in an ultra-small 7.9- by 3.6- by 1.5-mm package. The small footprint is useful in space- and height-constrained applications like point-of-load telecommunications and test-and-measurement power supplies.

Texas Instruments’ latest, compact DC-DC power module can handle 6A with only two external capacitors.

The synchronous step-down DC-DC converter output current is up to 6 A, provided by two phases that handle up to 3 A each. The two run out-of-phase, thereby significantly reducing pulse current noise. The TPSM82480 can automatically enter a power save mode to maintain high efficiency with very light loads. This is done using the automatic phase adding and shedding feature for both or only one phase based on actual load. The power save mode can be disabled.

The input voltage range is 2.4 to 5.5 V. This allows operation with supply sources that are typically 3.3-V or 5-V. It can handle backup circuits dropping as low as 2.4 V. The power module is able to operate in 100% duty cycle mode. The system has undervoltage lockout and over temperature protection.

The TPSM82480 provides a Power Good signal as well as an adjustable soft start. The device features a Thermal Good signal to indicate excessive internal temperature. The output voltage can also be changed to a preselected value using the VSEL pin.