A new generation of temperature sensor ICs with ±0.4C accuracy in a 0.76mm2 package
Since entering the IC design era, integrated circuit (IC) temperature sensors have inadvertently become part of the device design. IC designers have experienced twists and turns trying to minimize the effects of temperature on the chip system. An IC designer suddenly had a wonderful idea: Why not actively exploit the temperature behavior of active circuit p-n junctions, rather than being limited to racking your brain to minimize its effects. Designers who integrate digital functions into the same chip are even more open-minded, and they are cultivating current temperature sensor ICs.
The integrated temperature sensor easily solves most temperature sensing challenges in the temperature range of -55 to 200oC.
The input to the temperature sensor IC is the ambient temperature. In other words, the ambient temperature around the package changes the behavior of the internal transistor (Figure 1).
In Figure 1, we see how the difference between VBE and VBE(N) is a formula that easily corresponds to temperature changes.
(1) shows the value of the transistor base-emitter voltage VBE.
k is a Boltzmann constant equal to 1.38×10-23 J/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.
If the current source IC is switched from one pin to the other, Equation (3) shows the difference between the two base-emitter voltages.
Through calculation, it is concluded that:
CONSTANT=k × ln(N)/q or 86.25×10-6 × ln(N).
Conceptually, it lets you know how to measure temperature quickly at the IC level. A few improvements have been made to the circuit in Figure 1, with IC temperature sensing accuracy as high as ±0.4oC.
Now that we have accurate temperature readings, it is important to present this final value to the outside world. There are two basic ways to display temperature data: Analog voltage or digital value.
The analog voltage output is very easy to read. With the appropriate temperature sensing device, you can capture an analog signal and convert it to a digital representation or feed back to a point in the circuit.
The digital output capability of the temperature sensor is more interesting, with many output types available, but mainly 1-wire, 2-wire or 3-wire outputs.
The 2-wire digital output provides an I2C or SMBus signal. The digital result is a by-product of the internal analog to digital converter. You can also see digital outputs that represent threshold temperatures and possible error conditions.
The 3-wire digital output provides an SPI interface.
Wafer-level packaging of temperature sensor devices
Every product has a development process from rough to fine, and the temperature sensor series is constantly improving. This product line will next have a huge breakthrough in the size of the device package. The latest temperature sensor housings are packaged in a wafer level package (WLP).
In 1998, Sandia National Laboratories and Fujitsu developed WLP. The package has been wafer level fabricated prior to the dicing process and its assembly is done using standard surface mount technology (SMT).
This packaging technology brings an ultra-small package shape and low θ junction-environment values. The size of this generation of temperature sensors dwarfs the standard 0.1μF capacitor in a standard 0603 package (Figure 2).