IC Temperature Sensors Find th

Abstract: This applicaTIon note discusses the operaTIng concepts behind temperature sensor ICs and introduces the MAX1617, the first temperature sensor IC to measure the temperature of a remote thermal diode which allows accurate monitoring of the die temperature of another IC or discrete transistor.

Real-TIme temperature measurements ensure that today's smaller and faster systems operate in the safe thermal zone. The newest IC temperature sensors monitor external- and internal-component hot spots with pinpoint accuracy.

IC temperature sensors have come of age. Driven by PC and automoTIve applications, designers have embedded these ubiquitous heat sniffers in almost every electronic system larger than a pager. Cellular phones usually include one or more sensors in the battery pack, and notebook computers might have four or more sensors for checking temperatures in the CPU, battery, AC adapter, and PCMCIA card cage. Consequently, the design and manufacture of IC temperature sensors has become a $300 million/year industry.

These applications do not cover the enormous number of thermal-shutdown and -protection circuits that designers build into all sorts of ICs as a final defense against short circuits and over-clocking (exceeding the IC's specified clock speed). They cannot always replace the traditional temperature sensors—resistance temperature detectors, thermistors, and thermocouples—but IC temperature sensors offer many advantages. They require no linearization or cold-junction compensation, for instance. Indeed, they often provide cold-junction compensation for thermocouples. They generally provide better noise immunity through higher-level output signals, and some provide logic outputs that can interface directly to digital systems. Rube Goldberg Contraptions A discussion of IC temperature sensors has become timely and important, because electronic systems are increasingly dense, power-hungry, and hot. Temperature sensors also have a lot of gadget appeal. Many ICs perform highly abstract functions. (Look! It's a quadrature-amplitude modulator!) But temperature sensors relate directly to the real world that clicks, whirs, and hums. Put your finger on a temperature sensor, and it responds by actuating a fan or a buzzer. The more complex sensors often resemble a Rube Goldberg contraption with a digital interface—having autonomous operation and perhaps an ability to broadcast warning messages with an identifying return-address code that pinpoints the originator.

In the early days of ICs, IC temperature sensors were simple DIP devices that measured their own package temperature and generated a proportional output-voltage signal. Applications were easy: You simply ran the analog output signal into a voltage comparator or an A/D converter. Today, a proliferation of new devices provides remote sensing, airflow sensing, and other interesting features. This article surveys the IC temperature sensors available by type and provides guidelines for matching them to applications and making a trade-off among their specifications and features.

A Temp Sensor in Every Bandgap The ΔVBE bandgap reference is the heart of nearly all IC temperature sensors. First, the term "bandgap" is something of a misnomer: It refers to the bandgap voltage of silicon, which is 1.12V at room temperature. By sheer coincidence, this value nearly equals the magic voltage at which a negative-temperature-coefficient (TC) VBE, summed with a positive-TC-canceling voltage, results in a stable, zero-TC reference.

The forward voltage of a silicon pn junction is

VBE = VG0(1-T/T0)+VBE0(T/T0)+(nKT/q)ln(T0/T)+(KT/q)ln(IC/IC0),

where T is the temperature in degrees Kelvin, VG0 is the semiconductor bandgap extrapolated to absolute zero, VBE0 equals VBE at temperature T0 and corresponding current IC0, K is Boltzmann's constant, q is the charge of an electron, and n is a constant related to the device structure. Evaluating this equation at two current densities gives a simplified expression for the resulting ΔVBE:

ΔVBE = (KT/q)ln(IC1/IC2).

Thus, the difference in forward voltage is directly proportional to temperature. With accurate forcing of the two current levels, you can calculate temperature from a measured ΔVBE almost without regard to the initial forward voltage, physical size of the junction, leakage, or other junction characteristics. This principle underlies one of the most widely used IC cells in history, the Brokaw bandgap reference (Figure 1). You find this design or its close relative as part of the bias-current generator in the startup circuit of nearly every IC ever made—digital or analog.


Figure 1. All bandgap circuitry includes an electronic thermometer. In a MAX675 precision reference, the thermometer is accessible via a package pin. In other devices, it connects to a comparator, forming an emergency thermal-shutdown circuit.

The technique calls for forcing different current densities through the two transistors that form the heart of the reference. Though a discrete-component version, the bandgap circuit is similar to monolithic-IC versions. The two transistors operate with a current-density ratio of precisely 16 to 1. As the feedback from precision op-amp IC1 balances the circuit, the resulting VBE voltage is impressed across R1.

As current in R1 flows to ground through R2, the voltage generated at the emitter of Q2 has a positive TC of 2.2mV/°C. Summed with Q2's VBE, this voltage produces a zero-TC voltage at the VREF output terminal. IC2 buffers and scales the positive-TC voltage (VTC) to provide a precise output of 10mV/°C. Thus, most ICs contain a thermometer, but it is often of dubious accuracy and IC designers rarely make it available for external use.

The excessive leakage currents characteristic of silicon pn junctions limits the temperature for IC-based sensors to about 200°C. As a rule of thumb, these currents double with every 10°C rise in temperature. Excessive leakage current causes malfunctions in bandgap references and signal-conditioning circuitry.

Major Classes of IC Temp Sensor Vendors classify IC temperature sensors according to the input source and output-signaling method. The temperature source to be measured is usually the IC's own package, but you can measure airflow with an on-chip heater that raises the package temperature above ambient, and you can measure remote temperature with a diode-connected transistor. On the output side, analog-output, thermostat-logic-output, and serial-digital-output signaling methods are in widespread use. Table 1 provides a sampling of temperature sensors.

The first IC temperature sensors were basic analog-output devices that generated a voltage or current proportional to temperature. They remain highly useful, especially in designing purely analog systems that can take advantage of the temperature indication's virtually infinite resolution.

Designers commonly use simple logic-output devices to control cooling fans and other thermostat applications. When the package temperature of the sensor crosses a preset threshold, the sensor's logic output changes state. These devices often have connections that let you adjust the threshold temperature and hysteresis band with external resistor dividers. Other devices internally fix the thresholds and hysteresis. These simple chips (for instance, Maxim's MAX6501 family) recently became available in small, low-cost packages, such as SOT-23.

IC temperature sensors are most effective when you integrate them as part of an ASIC. Older NiCd battery packs usually have an onboard thermistor—quite cost-effective at less than 25 cents—rather than an IC temperature sensor. Newer lithium-ion battery packs typically integrate the temperature sensor with the pack's protection IC, which also performs overcurrent protection, cell balancing, fuel gauging, and other tasks.



Table 1. Representative Temperature Sensors
Device Measures Output Interface Typical Package Comments Analog Devices AD590