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Reading Assignment:
A study of the temperature-dependence of a silicon diode chip will provide a basic understanding of the simplest IC sensor, the analog output temperature sensor, where a voltage or current proportional to temperature is generated. A simple temperature sensor is one that uses a diode, or a transistor designed to operate as a diode, that is driven by a current source. See equation below. VF for the diode or VBE for a transistor is also referred to as the Temperature Coefficient (TC) or tempco. VBE has a -2mV/°C slope.
VF = forward voltage across diode
k = 1.38 x 10-23 Joules/°K, Boltzmann constant
T = absolute junction temperature in degrees Kelvin
q = 1.6 x 10-19 Coulombs, electron charge
IF = forward current
IS = saturation current
Another approach is measuring the difference in the voltage when using the junctions of two diodes with different current densities. The output voltage is proportional to the absolute temperature. The differential output voltage is typically very small in the order of 1mV/°K, so an amplification circuit is usually used to create a more convenient temperature coefficient such as 10mV/°K. This is one of the oldest techniques used for measuring temperature. Typically, room temperature is 25°C and 298°K.
The Kelvin precision temperature sensors, LM135, LM235, LM335, have a tempco of +10mV/°K. The only difference between each of these sensors is the temperature ranges over which they operate. The output voltage is determined simply by the equation below.
Vout = °K x 10mV/°K
Some applications require an output voltage that is proportional to the Centigrade or Fahrenheit temperature. These sensors are LM35 (Centigrade) and LM34 (Fahrenheit). The tempco for the LM35 is +10mV/°C and includes an offset of -2.73V to Centigrade measurement. The output voltage is determined, therefore, by the equation below.
Vout = °C x 10mV/°C
The tempco for the LM34 is +10mV/°F. The output voltage is determined by the equation below.
Vout = oF x 10mV/°F
Package Size and Supply Voltage Constraints
As IC temperature sensors found more applications, package size and supply voltages became important issues. Several sensors were developed to fit smaller form-factors while operating at lower supply voltages and/or with single or multiple power supplies. By changing some of the parameters, such as the operating voltage ranges, tempco, and offset voltages, many additional families of temperatures sensors were created to fit a variety of applications.
Additional Resources:
To see the different types of analog temperature sensors, read National Semiconductor's "Temperature Sensor Handbook," page 5-6.
To see typical circuit layouts and functionality of analog temperature sensors, read AN-460, "LM35/LM45 Precision Monolithic Temperature Sensors."
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Emmy Denton is a staff engineer in National Semiconductor's Data Conversion Systems Group in Santa Clara CA working with temperature sensors and hardware monitors.