ic temperature sensors jared bench ece 5320 spring 2004
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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
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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
IntroductionIntroduction
Limitations & Limitations & AdvantagesAdvantages
Working Working PrinciplesPrinciples
Sensor Types Sensor Types
InterfacingInterfacing
ApplicationsApplications
ConclusionsConclusions
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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/
IntroductionIntroduction
Limitations & Limitations & AdvantagesAdvantages
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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.
IntroductionIntroduction
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Desired Sensor Characteristics
Accuracy Practicality Reliability High Precision Easy to Interface Linearity Wide Range
IntroductionIntroduction
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Common Temperature Sensors
Thermistors Resistance Temperature Detectors
(RTDs) Infrared (IR) Thermocouples IC Temperature Sensors
IntroductionIntroduction
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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.
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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.
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Linearity IC Temp Sensors are highly linear.
The linearity of thermistors and Si temp sensors is contrasted below.
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Image: Maxim Integrated Products
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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.
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Principle of IC Temp Sensors
Operation of IC temperature sensors is based on the behavior of silicon PN junctions as a function of temperature.
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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)
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IC Temperature Sensor Theory
V1 - V2 = (kT/q) ln(J1/J2) Thus the difference in forward
voltage is directly proportional to absolute temperature
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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
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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.
IC Temperature Sensor Theory
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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.
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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 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
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Analog Sensors An ideal analog sensor provides an
output voltage that is a perfectly linear function of temperature
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Images: Maxim Integrated Products
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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.
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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.
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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.
IntroductionIntroduction
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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.
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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 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”.
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CPU Temperature Sensor
A user-programmable temperature sensor monitors the temperature of a remote CPU's on-chip PN junction.
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Image: Maxim Integrated Products
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Remote Temperature Sensor This user programmable temperature sensor
can monitor its own local temperature and the
temperatures of four remote PN junctions.
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Image: Maxim Integrated Products
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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.
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Image: Maxim Integrated Products
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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.
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Digital Fan Controller A fan controller/temperature sensor IC can use
either a PWM or a linear mode control scheme. IntroductionIntroduction
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Image: Maxim Integrated Products
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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.
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Image: Maxim Integrated Products
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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.
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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
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