Understanding Smart Sensors - Nomads.usp

Mechatronics and Sensing Systems 253Sensing for fault conditions, such as a short circuit, is an integral part ofmany smart-power ICs. The ability to obtain temperature sensors in a mixedbipolar-CMOS-DMOS semiconductor process provides protection and diagnosticsas part of the features of smart-power ICs. The primary function of thepower IC is to provide a microcontroller-to-load interface for solenoids, lamps,and motors. In multiple-output devices, sensing the junction temperatureof each device allows the status of each device to be input to the MCU,and, if necessary, the MCU can shut down a particular unit that has a faultcondition [3].In ICs, the temperature limit can be sensed by establishing a band gap reference.Two diodes’ junctions are biased with different current densities, butthe ratio is essentially constant over the operating temperature range (−40°Cto150°C). The following equation shows how the differential voltage (∆V be )isrelated to the current (I ) and emitter area (A) of the respective transistors:I1 A1Vbe 1− Vbe 2= Vbe = [ kBT q]⋅⎛ /⎜⎞∆ / ln ⎟⎝ I2/A2⎠(11.1)where k B is Boltzman’s constant (1.38 ⋅ 10 −23 J/K), T is the temperature in K,and q is the charge of an electron (1.6 ⋅ 10 −19 C).At a targeted high temperature, the negative temperature coefficient of V beequals the positive temperature coefficient voltage developed across a senseresistor, and a thermal limit signal is generated. Other circuitry regulates thevoltage and turns off the output power device.A smart-power IC can have multiple power drivers integrated on a singlemonolithic piece of silicon. Each driver can have a temperature sensor integratedto determine the proper operating status and shut off only a specificdriver if a fault occurs. Figure 11.2 shows an eight-output driver that independentlyshuts down the output of a particular driver if its temperature is excessive(between 155 and 185°C).The octal serial switch (OSS) adds thermal sensing through overtemperature-detectioncircuitry to the protection features. Faults can be detectedfor each output device and individual shutdown can be implemented. In amultiple-output power IC, it is highly desirable to shut down only the devicethat is experiencing a fault condition and not all the devices that are integratedon the power IC. With outputs in various physical locations on the chip, it isdifficult to predict the thermal gradients that could occur in a fault situation.Local temperature sensing at each output, instead of a single global temperaturesensor, is required.