Small, lower power temperature sensors: A cooling trend

Article By : Bonnie Baker

IC temperature sensors are now smaller than capacitors.

I am always looking for temperature measurement ICs that support applications over the entire electrical equipment spectrum. Once I get past the accuracy and temperature specifications, I concentrate on IC sensors that are usable in wearable applications and consumer mobile devices. But I am most interested in aggressively minimizing my PCB real estate and power ratio. The smaller, the better for most wearable and mobile applications.

The evolution in electronic circuit package sizes has gone from metal can to plastic dual-inline (PDIP) packages, to SOT23, and then to even smaller plastic encasements. I actually prefer leaded packages since they make my breadboard work so much easier. However, for the most part, encapsulated IC packaging is becoming more and more the norm.

The encapsulation process includes chips that are still part of a wafer. This new packaging type, called a wafer-level package or WLP, involves encapsulating the entire wafer, adding bumps on the bottom, and dicing the wafer’s individual chips. With WLPs, the end packaged product is as close as you can get to the actual die size. Now, IC designers have yet another design objective, to minimize the size of the die.

Table 1 shows a comparison of typical digital-out I2C temperature sensor packages with their dimensions and total PCB area.

temperature sensors comparisonTable 1 Comparison of four small digital-out temperature sensors

In Table 1, device A is a low-power I²C WLP temperature sensor. The WLP surpasses the size capability of the other temperature sensor packages. The size difference between a standard capacitor package (0603) and a WLP is astounding (Figure 1).

SMT 0.1 µF capacitorFigure 1 An SMT 0.1 µF capacitor (C1) is 73% larger than the WLP temperature sensor (U1).

Keep the power low

Along with smaller-sized IC components, power reduction is another benefit from utilizing WLPs. Although these two specifications are not correlated, smaller packages are a great match for portable, wearable, and battery-operated products.

When it comes to power, the simple calculation of volts times current provides the watts expended, right? Well, yes, under normal conditions, but battery-powered applications are different. The simple calculation now becomes amperes over time. Why? Because battery specification is measured in milliampere-hours (mA-Hr).

So, the voltage specification for the ICs only matters in terms of being compliant with the battery voltage. That is all. The key to this power evaluation is to determine the ampere consumption during the time that the temperature sensor is operating.

The amount of current a digital-out temperature sensor pulls from the battery occurs under two conversion conditions: one-shot or automatic continuous.

For a 12-bit temperature sensor, the automatic continuous conversion rate bits can be 0.25sps, 1sps, and 4sps conversion rates. In this mode, a conversion starts every 0.25s, 1s, or 4s, with the shutdown mode between conversions. This temperature sensor produces a 10-bit output at 0.25 conversions per second, which requires an average draw of 1.2 µA (Figure 2).

MAX31875 typical supply currentFigure 2 MAX31875 typical supply current (µA) vs. resolution (bit)

In Fig. 2, if you have a 12-bit output with 4 conversions per second, it uses an average of 45 µA.

Temperature measurement circuits commonly appear in sensor systems ranging from industrial automation to wearable devices. The challenge of these temperature systems is to have a temperature sensor in the smallest package with lower power. The electrical temperature sensor becomes a shoo-in for this type of application with small WLP housing and extremely low power performance.

Bonnie Baker has been working with analog and digital designs and systems for more than 30 years.