IC Temperature Sensors Jared Bench ECE 5320 Spring 2004

Contents Why IC Temperature Sensors Limitations/Advantages Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Why IC Temperature Sensors Limitations/Advantages IC Temperature Sensors Explained Types of IC Sensors Interfacing Applications Conclusions

References Claire O'Keeffe and Brian Black, “Choosing an IC Temperature Sensor” http://www.electronicproducts.com/ “IC Temperature Sensors Find the Hot Spots”, http://www.maxim-ic.com/ Jay Scolio, “Temperature Sensor ICs Simplify Designs”, Maxim Integrated Products, Inc. MAX675 Data Sheet, Maxim Integrated Products, Inc. “National Semiconductor’s Temp Sensor Handbook”, www.national.com/appinfo/tempsensors/files/temphb.pdf “National Semiconductor’s Analog University”, www.national.com/AU/design/1,4678,18_0_,00.html Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions

Further Reading List of Vendors, http://temperatures.com/semivendors.html “Introduction to Semiconductor Temperature Sensors”, www.capgo.com/Resources/ Temperature/Semiconductor/Semi.html “Sensor Industry Developments and Trends”, Sensor Business Digest, September 2004 Tons of Temperature Sensor Information, http://temperature-measurement.globalspec.com/ Computer Interfacing Example, http://www.iguanalabs.com/tempture.htm http://www.temperatures.com/forum/ Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions

Why Monitor Temperature Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Monitoring Temperature is becoming more important as electronic systems become increasingly dense and power-hungry. Systems are affected by temperature extremes Components may be damaged if the temperature falls outside the operating range.

Desired Sensor Characteristics Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Accuracy Practicality Reliability High Precision Easy to Interface Linearity Wide Range

Common Temperature Sensors Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Thermistors Resistance Temperature Detectors (RTDs) Infrared (IR) Thermocouples IC Temperature Sensors

IC Temp Sensors at a Glance Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Varied array of functions, features, and interfaces. Capabilities: Report both local and remote temperatures Monitor other system parameters Control fans Warn when a specific temperature is exceeded.

IC Temp Sensor Advantages Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Small and simple Accurate Inexpensive No linearization or cold-junction compensation is required. External or internal hot spots can be monitored. Generally provide better noise immunity through higher-level output signals. Easy to interface with other devices such as amplifiers, regulators, DSPs, and micro-controllers.

Linearity IC Temp Sensors are highly linear. The linearity of thermistors and Si temp sensors is contrasted below. Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Image: Maxim Integrated Products

IC Temp Sensor Limitations Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions The use of IC temperature sensors is limited to applications where the temperature is within a –55° to 150°C range.

Principle of IC Temp Sensors Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Operation of IC temperature sensors is based on the behavior of silicon PN junctions as a function of temperature.

IC Temperature Sensor Theory Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions A current is forced through two PN junctions with different active areas. The difference between the forward voltages on the two junctions is proportional to absolute temperature: V1 - V2 = (kT/q) ln(J1/J2)

IC Temperature Sensor Theory Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions V1 - V2 = (kT/q) ln(J1/J2) Thus the difference in forward voltage is directly proportional to absolute temperature

IC Temperature Sensor Theory Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions With accurate forcing of the two current levels, temperature can be calculated from a measured VBE almost without regard to the initial forward voltage, physical size of the junction, leakage, or other junction characteristics.

IC Temperature Sensor Theory Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions The excessive leakage currents characteristic of silicon PN junctions limits the temperature for IC-based sensors to about 200°C. These currents double with every 10°C rise in temperature, causing malfunctions in bandgap references and signal-conditioning circuitry.

Types of IC Temp Sensors Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Classified according to the input source and output-signaling method. Two main types of IC temperature sensors: Analog Produce a voltage or current proportional to temperature Digital Include an integrated A/D converter Can include other features such as voltage monitoring, fan control, and over or under limit alarms.

Representative Temperature Sensors Device Measures Output Interface Package Comments Analog Devices AD590 Package temperature Analog current SO-8 Very stable, immune to line-voltage drops in remote sensing, good noise immunity Maxim MAX675, REF-01, LM45, Analog Devices AD22103 Analog voltage SO-8 or SOT-23 Often combined with a voltage reference or other building blocks, shunt and buffered-VOUT types available TMP01, TC620, Maxim MAX6502 Thermostat logic output SOT-23 Built-in analog comparators, usually with adjustable hysteresis Dallas Semiconductor DS1621, National Semiconductor, LM75 and LM78, Linear Technology LT1392 Serial digital interface SO-8, SO-16 I2C, SPI, SMBus interfaces; sometimes built into large, multifunction A/D-converter ICs Maxim MAX1617 Remote diode junction 16-pin QSOP SMBus interface; monitors CPU temperatures directly Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions

Analog Sensors An ideal analog sensor provides an output voltage that is a perfectly linear function of temperature Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Images: Maxim Integrated Products

Analog Plus Sensors Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions "Analog Plus" sensors are available with various types of digital outputs. The "plus" added to the analog temperature sensor can be a comparator such that logic output trips when temperature passes a preset threshold. Other types of "plus" sensor relay temperature data in the form of the delay time after the part has been strobed, or in the form of the frequency or the period of a square wave.

Digital I/O Sensors Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Digital temperature data is passed to the microcontroller, usually via a serial bus. Data is sent to the temperature sensor from the microcontroller along the same bus, usually to set the temperature limit at which the alert pin's digital output will trip. An interrupt is sent to the microcontroller when the temperature limit has been exceeded. May also provide fan control.

System Monitor Sensors Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions In addition to the functions provided by the digital I/O type, this type of device commonly monitors the system supply voltages, providing an alarm when voltages rise above or sink below limits set via the I/O bus. Fan monitoring and/or control can also be included in this type of IC. In some cases, this class of device is used to determine whether a fan is working. More complex versions control the fan as a function of one or more measured temperatures.

Types of Interfaces Interfaces include: Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Interfaces include: Single wire Pulse Width Modulation (PMW) Two wire I2C and SMBus Used mostly in PC and communications applications Three or four wire SPI protocols. Serial interface common in non-PC environments Can also be bi-directional Microcontroller gives instructions to the temperature sensor.

Applications Remote Temperature Monitor Measures two temperatures Local Temperature Temperature of a remote location using a diode connected transistor Very useful in space constrained applications May be integrated on a microprocessor chip Eliminates the inaccuracy that results from a thermal resistance path between the sensor and chip. Facilitates the ability to increase clock speed without exceeding thermal capabilities. Can be used as a sensor for clock throttling. Used in “almost every electronic system larger than a pager”. Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions

CPU Temperature Sensor A user-programmable temperature sensor monitors the temperature of a remote CPU's on-chip PN junction. Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Image: Maxim Integrated Products

Remote Temperature Sensor This user programmable temperature sensor can monitor its own local temperature and the temperatures of four remote PN junctions. Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Image: Maxim Integrated Products

Distributed Temperature Sensing Here a microcontroller strobes eight temperature sensors connected on a common line and receives the temperature data transmitted from each sensor on the same line. Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Image: Maxim Integrated Products

Applications Monitoring systems Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Monitoring systems Systems requiring thermal monitoring and control may require monitoring of other system parameters Power supply Internal voltages and currents DC offsets of critical signals.

Digital Fan Controller A fan controller/temperature sensor IC can use either a PWM or a linear mode control scheme. Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Image: Maxim Integrated Products

Analog Plus Heater Controller A temperature sensor can transmit a square wave whose frequency is proportional to temperature as part of a heater controller circuit. Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions Image: Maxim Integrated Products

Digital Over-Temp Indicator Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions This sensor can signal when a temperature has been exceeded Well suited for: Over or under-temperature alarms On/off fan control. Image: Maxim Integrated Products

Conclusions IC Temperature Sensors: Are small and inexpensive Introduction Limitations & Advantages Working Principles Sensor Types Interfacing Applications Conclusions IC Temperature Sensors: Are small and inexpensive Have desirable sensor characteristics Are available in many interface types Can be easily integrated into existing systems Have many practical applications