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Maxim > Design Support > Technical Documents > Application Notes > Temperature Sensors and Thermal Management > APP 694
Keywords: temperature sensor, ICs, analog, digital, temp sensors, remote diode, thermal diode, temperature-
to-frequency converters, temperature-to-time, temperature switch, temp sensor ICs
APPLICATION NOTE 694
Temperature Sensor ICs Simplify Designs
By: Kerry Lacanette
Jan 31, 2001
Abstract:
When
you set out to select a temperature sensor, you are no longer limited to either an analog
output or a digital output device. There is now a broad selection of sensor types, one of which should match
your system's needs. This application note surveys the wide variety of temperature sensor ICs available to
help the designer make the best choice for a particular design.
When you set out to select a temperature sensor, you are no longer limited to either an analog-output or a
digital-output device. There is now a broad selection of sensor types, one of which should match your
system's needs.
When asked what I do for a living, I tell people I'm an analog electronics engineer. When next asked, "What
does 'analog' mean?" I often use temperature sensors to explain the concept. I tell them that we engineers
sometimes use sensors that give an analog of temperature in the form of voltage or current. To further define
the analog world, I contrast it with the digital realm by saying that the voltage or current signals are usually
converted to 1s and 0s so that computers can use the information. Sometimes this explanation suffices;
sometimes it doesn't.
What this explanation does do is suggest the basic migration of temperature sensors. Until recently, all the
temperature sensors on the market provided analog outputs. Thermistors, RTDs, and thermocouples were
followed by another analog-output device, the silicon temperature sensor. In most applications, unfortunately,
these analog-output devices require a comparator, an ADC, or an amplifier at their output to make them
useful.
Thus, when higher levels of integration became feasible, temperature sensors with digital interfaces became
available. These ICs are sold in a variety of forms, from simple devices that signal when a specific
temperature has been exceeded to those that report both remote and local temperatures while providing
warnings at programmed temperature settings. The choice now isn't simply between analog-output and
digital-output sensors; there is a broad range of sensor types from which to choose.
Classes of Temperature Sensors
Four temperature-sensor types are illustrated in Figure 1. An ideal analog sensor provides an output voltage
that is a perfectly linear function of temperature (A). In the digital I/O class of sensor (B), temperature data in
the form of multiple 1s and 0s are passed to the microcontroller, often via a serial bus. Along the same bus,
data are sent to the temperature sensor from the microcontroller, usually to set the temperature limit at which
the alert pin's digital output will trip. Alert interrupts the microcontroller when the temperature limit has been
exceeded. This type of device can also provide fan control.
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