System Description
Figure 1 shows a circuit diagram of a remote temperature sensing system. The circuit includes the TMP468 device that monitors nine temperature zones using the internal (local) temperature sensor, and eight remote thermal diodes with the associated filter circuitry (RS1, RS2, and CDIFF). Shown in the diagram is a two-wire I 2C or SMBus compatible system management controller that reports the zone temperatures from the TMP468 to a console, or coordinates system cooling and protection. Figure 1 shows overtemperature shutdown circuitry that takes direct hardware action to shut down the system based on the THERM outputs of the TMP468. The TMP468 includes two limits for each temperature zone that are associated with the THERM and THERM2 outputs. These limits digitally compare to the current temperature readings and activate the THERM and THERM2 outputs.
Thermal Diode Overview
The base emitter junction of a BJT has a very predictable transfer function that is dependent on temperature. Remote-junction temperature sensors use this principle to measure the temperature of an external transistor. There are four possible BJT configurations, as shown in Figure 2. The selected configuration is typically dependent on availability and the type of task. In most cases, remote junction temperature sensors measure the junction temperature of another device, such as a high-power processor, FPGA, or ASIC. The most common bipolar transistor found in CMOS processes is a substrate PNP with a collector that is tied to ground or substrate. To measure the case temperature of a device or board temperature, use a discrete transistor, as shown in Figure 2. Complicated systems require robust thermal management solutions with several temperature zones for protection and prevention of thermal runaway. Thus, the TMP468 measures the junction temperature of multiple highly integrated devices, and the temperature of stand-alone transistor junctions placed throughout the system to generate a thorough temperature profile.