ic temperature sensors

IC Temperature Sensors

Jared Bench

ECE 5320

Spring 2004

22/32/32 IC Temperature Sensors IC Temperature Sensors Jared BenchJared Bench

Why IC Temperature Sensors Limitations/Advantages IC Temperature Sensors Explained Types of IC Sensors Interfacing Applications Conclusions

Contents

IntroductionIntroduction

Limitations & Limitations & AdvantagesAdvantages

Working Working PrinciplesPrinciples

Sensor Types Sensor Types

InterfacingInterfacing

ApplicationsApplications

ConclusionsConclusions


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 IntegratedProducts, 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









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/









Why Monitor Temperature

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

Accuracy Practicality Reliability High Precision Easy to Interface Linearity Wide Range









Common Temperature Sensors

Thermistors Resistance Temperature Detectors

(RTDs) Infrared (IR) Thermocouples IC Temperature Sensors









IC Temp Sensors at a Glance

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

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.








Image: Maxim Integrated Products


IC Temp Sensor Limitations

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

Operation of IC temperature sensors is based on the behavior of silicon PN junctions as a function of temperature.









IC Temperature Sensor Theory

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)










V1 - V2 = (kT/q) ln(J1/J2) Thus the difference in forward

voltage is directly proportional to absolute temperature









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.










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

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.




Sensor TypesSensor Types





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

Package temperature 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

Package temperature Thermostat logic

output SOT-23

Built-in analog comparators, usually with adjustable hysteresis

Dallas Semiconductor DS1621, National Semiconductor, LM75 and LM78, Linear Technology LT1392

Package temperature 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 Serial digital

interface 16-pin QSOP

SMBus interface; monitors CPU temperatures directly









Analog Sensors An ideal analog sensor provides an

output voltage that is a perfectly linear function of temperature








Images: Maxim Integrated Products


Analog Plus Sensors

"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

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

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: 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”.









CPU Temperature Sensor

A user-programmable temperature sensor monitors the temperature of a remote CPU's on-chip PN junction.










Remote Temperature Sensor This user programmable temperature sensor

can monitor its own local temperature and the

temperatures of four remote PN junctions.










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.










Applications

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. IntroductionIntroduction









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.










Digital Over-Temp Indicator

This sensor can signal when a temperature has been exceeded

Well suited for: Over or under-

temperature alarms On/off fan control.










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








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