1. IC Temperature Sensors
Jared Bench
ECE 5320
Spring 2004

2. 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

3. References
Claire O'Keeffe and Brian Black, “Choosing an IC Temperature Sensor”
“IC Temperature Sensors Find the Hot Spots”,
Jay Scolio, “Temperature Sensor ICs Simplify Designs”, Maxim Integrated Products, Inc.
MAX675 Data Sheet, Maxim Integrated Products, Inc.
“National Semiconductor’s Temp Sensor Handbook”,
“National Semiconductor’s Analog University”,
Introduction
Limitations & Advantages
Working Principles
Sensor Types
Interfacing
Applications
Conclusions

4. Further Reading
List of Vendors,
“Introduction to Semiconductor Temperature Sensors”, Temperature/Semiconductor/Semi.html
“Sensor Industry Developments and Trends”, Sensor Business Digest, September 2004
Tons of Temperature Sensor Information,
Computer Interfacing Example,
Introduction
Limitations & Advantages
Working Principles
Sensor Types
Interfacing
Applications
Conclusions

5. 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.

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

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

8. 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.

9. 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.

10. 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

11. 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.

12. 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.

13. 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)

14. 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

15. 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.

16. 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.

17. 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.

18. 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

19. 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

20. 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.

21. 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.

22. 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.

23. 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.

24. 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

25. 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

26. 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

27. 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

28. 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.

29. 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

30. 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

31. 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

32. 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