Hot stuff: picking a temperature sensing solution for automotive applications

Thank an automotive engineer if you drive a car in a place that juxtaposes multiple major climate zones within a small driving range. Take California, for example, where temperatures in Death Valley reach +55°C and as low as -35°C within a few hours’ drive in Mono County. Just think of all the thermal tolerance that your car and its electronics must have in order to handle these wide temperature ranges – on top of the car’s normal operating extremes seen in body and under-the-hood applications.

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No wonder the automotive industry increasingly requires the reliability expected by AEC-Q100 qualified integrated circuit (IC) components. An electronic rearview mirror or tiny, sealed camera module that bakes in the sun all day might requireAEC-Q100 Grade 1components up to 125°C. An engine control unit under the hood increasingly needs Grade 0 components rated for 1,000 hours of operation up to 150°C – and sometimes that’s not even enough. The one component that must be accurate at these extreme temperatures in order to protect the system is no doubt the temperature sensor. Accurate temperature information allows the processor to temperature-compensate the system so that the electronic modules can optimize their performance and maximize their reliability no matter the driving conditions.

IC temperature sensors share a market category with other sensing technologies like thermistors, resistance temperature detectors (RTD) and thermocouples, but ICs have some important benefits when good accuracy is required over wide temperatures like the AEC-Q100 Grade 0 range (-40°C to 150°C). First, the accuracy limits of an IC temperature sensor are given in degrees Celsius in the data sheet across the full operating range; conversely, a typical negative temperature coefficient (NTC) thermistor may only specify the resistance accuracy in percent at a single temperature point. You would then need to carefully calculate the total system accuracy for the full temperature range when using a thermistor. In fact, be careful to check the operating conditions specifying any sensor’s accuracy. For example, generously specified ICs will give the accuracy over the supply-voltage range rather than only at one specific voltage.

Another benefit is that IC temperature sensors are highly linear, which minimizes the need for software compensation. Figure 1 is a linearity comparison of an analog IC temperature sensor and a thermistor.


Figure 1: Linearity comparison of an IC temperature sensor vs. a typical positive temperature coefficient (PTC) thermistor

When selecting an IC, keep in mind that there are several types – with various merits for different automotive applications.

  • Analog output. Devices like the LMT84 (available in AEC-Q100 Grade 0) are simple, three-pin solutions that offer multiple gain options to match best with your selected analog-to-digital converter (ADC), which lets you determine the overall resolution. You also get the benefit of low operating power consumption that is comparatively consistent over the temperature range vs. a thermistor. This means you don’t have to trade off power for noise performance. Analog is cost-effective solution for infotainment systems.
  • Digital output. A popular interface choice for under-the-hood applications is SPI. Devices like the LM71 (available in AEC-Q100 Grade 0) feature a 13-bit resolution in a small SOT-23 package, boasting the simple yet reliable SPI interface. Other digital devices such as the LM95172 offer up to 16 bits of resolution, additional features and package options rated to 200°C. Digital temperature sensors like these are popular in powertrain and chassis applications like transmission control and electronic braking. I2C devices like the Grade 1-qualified TMP102 can support infotainment and instrument-cluster applications when a tiny solution is needed.
  • Temperature switch. Many of TI’s automotive-qualified switches integrate the thermostat function plus analog output. They provide simple, reliable over-temperature warnings, but having the analog temperature value gives your system an early indicator that you can use to scale back to limited operation before getting to a critical temperature. Headlamps benefit from simple, reliable implementation of factory-preset thresholds with the LM26LV. Transmission control also benefits from the programmable thresholds, ultra-wide operating temperature range and high reliability from in-circuit operational verification of the LM57 due to the harsh operating environment (both ICs are available in AEC-Q100 Grade 0).
  • Remote diode. Advanced processing applications like infotainment systems and instrument clusters benefit from high-accuracy devices like the TMP411 and TMP451. These AEC-Q100 Grade 1-qualified sensors take an input from a substrate thermal transistor or diode in the CPU/FPGA or a remote diode-connected transistor strategically placed elsewhere on the board. They also integrate their own local temperature sensor and communicate over a standard two-wire interface.

TI’s extensive portfolio of AEC-Q100-qualified temperature sensor products includes devices with extended temperature range offerings. The new LM57-Q1 temperature switch, for example, can sustain operation profiles up to 170°C. This is critical when the system temperature profile requires operating at an extended high temperature.

TI is routinely adding new temperature sensors to our AEC-Q100-qualified device lineup, so watch for new additions.

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