The temperature sensor IC can easily solve most of the temperature range of -55 to 200ºC...

Since entering the era of IC design, integrated circuit (IC) temperature sensors have inadvertently become a part of device design. IC designers have gone through ups and downs, trying to minimize the impact of temperature on the chip system. Turning around, an IC designer suddenly had a brilliant idea: why not actively exploit the temperature behavior of active circuit pn junctions instead of confining to racking their brains to minimize its impact. The designers who integrate digital functions into the same chip are even more open-minded. It is they who gave birth to the current Temperature Sensor IC.

The integrated Temperature sensor can easily solve most of the temperature sensing problems in the temperature range of -55 to 200ºC.

Input

The input of the temperature sensor IC is the ambient temperature. In other words, the ambient temperature around the package changes the behavior of the internal transistors (Figure 1).

Figure 1: This conceptual circuit shows how matched transistors detect temperature.

The temperature sensing design eliminates the influence of transistor saturation current (IS) through clever configuration and calculation. It is easy to control the saturation current using a constant current source (IC) and a switch between a transistor and an equivalent transistor array. In Figure 1, we see how the difference between VBE and VBE(N) easily corresponds to temperature changes.

The formula (1) shows the value of the transistor base-emitter voltage VBE.

Formula 1)

among them:

k is Boltzmann's constant, equal to 1.38×10-23J/K;

q is equal to 1.6021765×10-19C;

T is the temperature in K.

Equation (2) shows the VBE(N) value of the base-emitter of many parallel transistors.

Formula (2)

If the current source IC is switched from one pin to another, the formula (3) shows the difference between the two base-emitter voltages.

Formula (3)

Through calculation, we get:

CONSTANT=k × ln(N)/q or 86.25×10-6 × ln(N).

Conceptually, it lets you know how to quickly measure temperature at the IC level. With a few improvements to the circuit in Figure 1, the IC temperature sensing accuracy can be as high as ±0.4ºC.

Output

Now that we have an accurate temperature reading, how to present this final value to the outside world is very important. There are two basic methods for displaying temperature data: analog voltage or digital value.

The analog voltage output is very easy to read. Using the appropriate temperature sensing device, you can capture the analog signal, convert it into a digital representation or feed it back to a certain point in the circuit.

The digital output capability of the temperature sensor is more interesting. There are many output types to choose from, but the main ones are 1-wire, 2-wire or 3-wire output.

The 1-wire digital output can provide pulse width modulation (PWM) pulse count signals or simple threshold/switch signals. Both of these signals are useful in fan control circuits. The 2-wire digital output provides I2C or SMBus signals. The digital result is a by-product of the internal analog-to-digital converter. You can also see the digital output representing the threshold temperature and possible error conditions. The 3-wire digital output provides an SPI interface.

Wafer-level packaging of temperature sensor devices

Each product has a development process from crude to refined, and the temperature sensor series are constantly improving. This product series will have a huge breakthrough in the size of the device package next. The shell of the latest temperature sensor device adopts wafer level packaging (WLP).

In 1998, Sandia National Laboratory and Fujitsu developed WLP. The package has been manufactured at the wafer level before the cutting process, and its assembly is realized by standard surface mount technology (SMT).

This packaging technology brings ultra-small package outlines and low θ junction-environmental values. The size of this generation of temperature sensors dwarfs the standard 0.1μF capacitors in a standard 0603 package (Figure 2).

Figure 2: The size of the WLP temperature sensor (U1/MAX31875) is smaller than the 0.1μF capacitor (C1) of the SMT.

At the dinner table

Because of the small size of the new package, you can place the temperature sensor on the pcb arbitrarily, just like you sprinkle salt and pepper on the noodles when you cook dinner. The latest generation of temperature sensors can achieve an accuracy of ±0.4ºC in a package that occupies only 0.76mm2 pcb area. So, what's the dish tomorrow?