Reducing the size of integrated circuits (ICs) is often viewed as little more than a path to more compact printed circuit boards (PCBs) or the ability to add functionality without increasing the form factor of a system. And while systems from wearable electronics to automotive cameras continue pushing the limits of physical form factors, integrating the temperature-sensing signal chain and reducing temperature sensor size can directly affect the accuracy and response times of temperature measurements. The higher accuracy and quicker thermal response time of temperature sensors ultimately contribute to more efficiency of the system being monitored and better protection from overheating.

Integration improves accuracy

Although the small size of a nonintegrated temperature sensor can enable closer placement to hotspots in a system and quicker thermal response, they also make systems more susceptible to errors caused by system tolerance and noise. A negative temperature coefficient (NTC) thermistor, one of the simplest forms of a temperature sensor, is an example. Thermistors are one of the smallest options for the pure sensing function, but these components require voltage or current biasing with a resistor network. Ultimately, the total measurement error becomes a function of the thermistor tolerance itself, the tolerance of the bias resistor and voltage-supply errors. This discrete signal chain also leaves the system susceptible to noise, which is especially true of a system using NTC thermistors, which have very small resistance changes as the temperature rises. Your choices are to deal with the error caused by noise in the system or pull in more components to filter the signal. Even after considering compounding errors, the ability to react to the accuracy of this signal chain depends on the analog-to-digital converter resolution. While TI’s TMP61 family of linear thermistors improves accuracy and size over traditional NTC thermistors, the challenges remain.

Regardless of the size of your end-application, the only way to ensure the most accurate temperature measurement is to integrate the full sensing signal chain. Not only does an integrated temperature sensor offer an accuracy specification that is directly applicable in the system, but it can also dramatically reduce overall system size by minimizing external circuitry and eliminating the PCB routing required for a discrete implementation.

For example, to protect the image sensor in an automotive camera, designers often connect a thermistor circuit to a comparator for temperature threshold detection. If the image sensor has a maximum operating temperature of 115°C but system temperatures can reach 125°C, you need a shutoff threshold to prevent permanent damage. Even at 100°C, the system can reduce processor speeds, image refresh time or other parameters in order to keep the temperature from getting to the point of a full shutdown.

Table 1 shows how the TMP392 integrated dual threshold switch offers 42% area savings vs. a discrete implementation using thermistors and a comparator. This area reduction also ensures temperature-sensing accuracy of ±1.5°C from 0°C to 70°C and ±3°C from –55°C to 130°C.

Table 1: Layout of a discrete dual-threshold detection circuit compared to the integrated TMP390 or TMP392 temperature switch

Size improves thermal response

While the integration of a temperature-sensing signal chain can minimize system-level errors and reduce PCB space, the accuracy and speed of the thermal response is also related to the thermal mass and placement of the sensing element. Thermal mass is a material’s ability to store heat energy. Because larger devices tend to have a larger thermal mass, it will take longer to absorb heat from the environment, and the system cannot react to thermal changes as quickly. This is a simple-enough concept, but it is complicated further by the potential paths for temperature change through the PCB or air around the sensor. Depending on your goals, you could isolate a component from the heat-generating components on the PCB or couple the sensor as closely to a heat-generating component as possible, as described in the application report, “Temperature Sensors: PCB Guidelines for Surface-Mount Devices.”

Beyond optimizing a design to measure the temperature of interest, engineers always have trade-offs between smaller sensing elements and higher levels of integration. Now, that decision is no longer necessary. TI’s TMP114 and TMP144 digital temperature sensors are available in a 0.758-mm-by-0.758-mm package, with a height of 0.15 mm, which is thinner than typical passive components such as NTC thermistors. Not only do these thin temperature sensors enable placement very close to heat-generating components, but as shown in Figure 1, the sensors are small enough to be placed between the solder balls of a processor for optimized thermal monitoring.

Figure 1: The TMP114 1.08- to 1.98-V digital temperature sensor mounted under a TI processor (view from side and bottom of the PCB rendering)

While smaller IC components certainly can contribute to a smaller system size, it’s the size of the temperature sensors that affects system performance. Instead of having to tradeoff between reacting quickly to temperature changes (to prevent system damage in the event of a short circuit or battery degradation) and maintaining the most accuracy (to extend the thermal limits of a processor or system in the context of the required thermal shutdown), now you can leverage the TMP114 and TMP144 to achieve both goals.

Additional resources

• Read the technical article, “How to Choose the Right Thermistor for Your Temperature Sensing Application.”
• Check out the application report, “System Size Reduction Through Integrated Solutions for Temperature Threshold Detection.”
• Read the application report, “Under-Component Monitoring with Ultra-Small Temperature Sensors.”