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IC Temperature

Sensors

Jared Bench

ECE 5320

Spring 2004


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2/32 IC Temperature Sensors Jared Bench

Why IC Temperature Sensors

Limitations/Advantages

IC Temperature Sensors Explained

Types of IC Sensors

Interfacing Applications

Conclusions

Contents

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions


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References

Claire O'Keeffe and Brian Black, Choosing an ICTemperature 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 Semiconductors Temp Sensor Handbook,www.national.com/appinfo/tempsensors/files/temphb.pdf

National Semiconductors Analog University,www.national.com/AU/design/1,4678,18_0_,00.html

Introduction

Limitations &

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Working

Principles

Sensor Types

Interfacing

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Conclusions


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Further Reading

List of Vendors,http://temperatures.com/semivendors.html

Introduction to Semiconductor TemperatureSensors, 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


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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.

Introduction

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Principles

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Interfacing

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Desired Sensor Characteristics

Accuracy

Practicality

Reliability

High Precision

Easy to Interface Linearity

Wide Range

Introduction

Limitations &

Advantages

Working

Principles

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Interfacing

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Conclusions


7/32 7/32 IC Temperature Sensors Jared Bench

Common Temperature Sensors

Thermistors

Resistance Temperature Detectors

(RTDs)

Infrared (IR)

Thermocouples

IC Temperature Sensors

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions



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.

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions



IC Temp Sensor Advantages

Small and simple

Accurate

Inexpensive

No linearization or cold-junction compensationis required.

External or internal hot spots can be monitored.

Generally provide better noise immunitythrough higher-level output signals.

Easy to interface with other devices such asamplifiers, regulators, DSPs, and micro-controllers.

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions



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

The use of IC temperature sensors

is limited to applications where the

temperature is within a55 to150C range.

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions



Principle of IC Temp Sensors

Operation of IC temperature

sensors is based on the behavior of

silicon PN junctions as a function oftemperature.

Introduction

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Working

Principles

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Interfacing

Applications

Conclusions


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IC Temperature Sensor Theory

A current is forced through two PN

junctions with different active areas.

The difference between the forwardvoltages on the two junctions is

proportional to absolute

temperature: V1 - V2 = (kT/q) ln(J1/J2)

Introduction

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Conclusions


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V1 - V2 = (kT/q) ln(J1/J2)

Thus the difference in forward

voltage is directly proportional toabsolute temperature

Introduction

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Working

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Interfacing

Applications

Conclusions


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With accurate forcing of the two

current levels, temperature can be

calculated from a measured VBEalmost without regard to the initial

forward voltage, physical size of the

junction, leakage, or other junctioncharacteristics.


Introduction

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The excessive leakage currents

characteristic of silicon PN

junctions limits the temperature forIC-based sensors to about 200C.

These currents double with every

10C rise in temperature, causingmalfunctions in bandgap references

and signal-conditioning circuitry.


Introduction

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Types of IC Temp Sensors

Classified according to the input sourceand output-signaling method.

Two main types of IC temperature

sensors: Analog

Produce a voltage or current proportional totemperature

Digital Include an integrated A/D converter Can include other features such as voltage

monitoring, fan control, and over or under limitalarms.

Introduction

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Working

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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

MAX6502Package temperature

Thermostat logic

outputSOT-23

Built-in analog comparators,

usually with adjustable

hysteresis

Dallas Semiconductor DS1621,National

Semiconductor, LM75

and LM78, Linear

Technology LT1392

Package temperatureSerial digital

interfaceSO-8, SO-16

I2C, SPI, SMBus interfaces;sometimes built into

large, multifunction

A/D-converter ICs

Maxim MAX1617 Remote diode junctionSerial digital

interface16-pin QSOP

SMBus interface; monitors

CPU temperatures

directly

Introduction

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Working

Principles

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Interfacing

Applications

Conclusions


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Analog Sensors An ideal analog sensor provides an

output voltage that is a perfectly linearfunction of temperature

Introduction

Limitations &

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Working

Principles

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Interfacing

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Conclusions

Images: Maxim Integrated Products


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Analog Plus Sensors

"Analog Plus" sensors are available withvarious types of digital outputs.

The "plus" added to the analog temperature

sensor can be a comparator such that logicoutput trips when temperature passes a presetthreshold.

Other types of "plus" sensor relay temperaturedata in the form of the delay time after the parthas been strobed, or in the form of thefrequency or the period of a square wave.

Introduction

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Digital I/O Sensors

Digital temperature data is passed to themicrocontroller, usually via a serial bus.

Data is sent to the temperature sensor

from the microcontroller along the samebus, usually to set the temperature limitat which the alert pin's digital output willtrip.

An interrupt is sent to the microcontrollerwhen the temperature limit has beenexceeded.

May also provide fan control.

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System Monitor Sensors

In addition to the functions provided by thedigital I/O type, this type of device commonlymonitors the system supply voltages, providing

an alarm when voltages rise above or sinkbelow limits set via the I/O bus.

Fan monitoring and/or control can also beincluded in this type of IC.

In some cases, this class of device is used todetermine whether a fan is working. Morecomplex versions control the fan as a functionof one or more measured temperatures.

Introduction

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Types of Interfaces

Interfaces include:

Single wire Pulse Width Modulation (PMW)

Two wire I2

C 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.

Introduction

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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 thermalresistance path between the sensor and chip.

Facilitates the ability to increase clock speed withoutexceeding thermal capabilities.

Can be used as a sensor for clock throttling.

Used in almost every electronic system larger than apager.

Introduction

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CPU Temperature Sensor

A user-programmable temperaturesensor monitors the temperature ofa remote CPU's on-chip PN

junction.

Introduction

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Working

Principles

Sensor Types

Interfacing

Applications

Conclusions

Image: Maxim

Integrated Products


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Remote Temperature Sensor

This user programmable temperature sensorcan 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


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Distributed Temperature Sensing

Here a microcontroller strobes eighttemperature sensors connected on a commonline and receives the temperature datatransmitted from each sensor on the same line.

Introduction

Limitations &

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Working

Principles

Sensor Types

Interfacing

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Conclusions

Image: Maxim Integrated Products


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Applications

Monitoring systems

Systems requiring thermal

monitoring and control may requiremonitoring of other system

parameters

Power supply

Internal voltages and currents

DC offsets of critical signals.

Introduction

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Principles

Sensor Types

Interfacing

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Digital Fan Controller

A fan controller/temperature sensor IC can useeither a PWM or a linear mode control scheme.

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions

Image: Maxim

Integrated Products


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Analog Plus Heater Controller

A temperature sensor can transmit a squarewave whose frequency is proportional totemperature as part of a heater controllercircuit.

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions

Image: Maxim

Integrated Products


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Digital Over-Temp Indicator

This sensor can

signal when a

temperature hasbeen exceeded

Well suited for:

Over or under-temperature alarms

On/off fan control.

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions

Image: Maxim Integrated Products


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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

Introduction

Limitations &

Advantages

Working

Principles

Sensor Types

Interfacing

Applications

Conclusions

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