IC Temperature Sensors

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IC Temperature Sensors

• Jared Bench
• ECE 5320
• Spring 2004

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Contents
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions

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

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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 ICsSimplify
  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 Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions
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Further Reading

• List of Vendors, http//temperatures.com/semivendo
  rs.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.iguanalab
  s.com/tempture.htm
• http//www.temperatures.com/forum/

Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions
5
Why Monitor Temperature
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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Desired Sensor Characteristics
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions

• Accuracy
• Practicality
• Reliability
• High Precision
• Easy to Interface
• Linearity
• Wide Range

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Common Temperature Sensors
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions

• Thermistors
• Resistance Temperature Detectors (RTDs)
• Infrared (IR)
• Thermocouples
• IC Temperature Sensors

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IC Temp Sensors at a Glance
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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IC Temp Sensor Advantages
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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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 Applicatio
ns Conclusions
Image Maxim Integrated Products
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IC Temp Sensor Limitations
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions

• The use of IC temperature sensors is limited to
  applications where the temperature is within a
  55 to 150C 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.

Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions
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IC Temperature Sensor Theory
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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)

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IC Temperature Sensor Theory
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions

• V1 - V2 (kT/q) ln(J1/J2)
• Thus the difference in forward voltage is
  directly proportional to absolute temperature

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IC Temperature Sensor Theory
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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IC Temperature Sensor Theory
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions

• The excessive leakage currents characteristic of
  silicon PN junctions limits the temperature for
  IC-based sensors to about 200C.
• These currents double with every 10C rise in
  temperature, causing malfunctions in bandgap
  references and signal-conditioning circuitry.

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Types of IC Temp Sensors
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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Representative Temperature Sensors
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions
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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 Applicatio
ns Conclusions
Images Maxim Integrated Products
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Analog Plus Sensors
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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Digital I/O Sensors
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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System Monitor Sensors
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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Types of Interfaces
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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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.

Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions
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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 Applicatio
ns Conclusions
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.

Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions
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.

Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions
Image Maxim Integrated Products
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Applications
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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.

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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 Applicatio
ns Conclusions
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.

Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns Conclusions
Image Maxim Integrated Products
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Digital Over-Temp Indicator
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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
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Conclusions
Introduction Limitations Advantages Working
Principles Sensor Types Interfacing Applicatio
ns 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