1 Introduction

When monitoring temperature of critical components for a system, greater accuracy results in better information feedback to temperature control loops, whether they are for compensation or for safety shutdown performance. In the case of monitoring processor or MCU temperature, designers often include a thermal diode pin, which allows for an external temperature monitoring IC to measure the internal temperature of the die. This method works well when both an internal diode is available, and when the diode is well constructed and has an over-temperature behavior which closely follows that of a standard BJT.

In some cases however, an integrated diode is not designed in, or the overtemperature characteristics render it either unusable or of very-poor quality for temperature detection. In such instances a designer can choose to use the internal temperature sensor of their processor if one is available. The primary drawback of the included sensing element in most modern processors is the extremely low accuracy, typically on the order of ± 5 to 10 °C.

Figure 1-1 SMT Processor Temperature Measurement Options

A better alternative is to use an external integrated temperature sensor or thermistor, to provide the best possible accuracy. Figure 1-1 shows several potential options for measuring the temperature of a processor. Option 1, is to use the internal diode or temperature sensor. In option 2, the sensor, either in a through-hole type package, a probe, or on a flex cable, is affixed to a heat sink using a thermal epoxy. A common variant of this is to place the sensor directly into a machined or drilled hole of a processor heat sink. Option 3 is to place a temperature sensor (IC or thermistor) elsewhere on the PCB, as close as possible to the Processor/MCU. Finally option 4, is to place a temperature sensor directly under the component you want to monitor. Previously, option 4 was only really feasible in applications which included a socket that provided adequate vertical clearance. The newest ultra-thin temperature sensor offerings from TI are discussed and can enable this method even in applications where the component is surface mounted.