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Maxim > Design Support > Technical Documents > Application Notes > A/D and D/A Conversion/Sampling Circuits > APP 1944
Maxim > Design Support > Technical Documents > Application Notes > Temperature Sensors and Thermal Management > APP 1944
Keywords: temperature, thermometer, diode, thermistor, thermister, ADC, die temp, temp, system monitor,
diode sensor, analog to digital converter
APPLICATION NOTE 1944
Temperature Monitoring Using the MAX1253/54 and
MAX1153/54
Mar 21, 2003
Abstract: The MAX1253/54 and MAX1153/54 system monitors are a low cost solution for monitoring multiple
temperatures in a system. This application note explains how to measure temperature using remote sensing
diodes in single-ended or differential configuration and possible error sources.
Remote temperature monitoring traditionally has been accomplished with thermistors, thermocouples or
temperature sensor ICs. If a system requires temperature measurements in multiple locations, the costs can
quickly add up. The MAX1253/1254 and MAX1153/1154 provide a low-cost solution to monitor multiple remote
temperatures as well as a single local temperature. In addition, these devices offer autonomous alarming
functions without any interaction required by a host controller, generating an interrupt when a user
programmable upper or lower threshold is exceeded, resulting in fully stand-alone temperature monitoring.
These IC devices monitor both arbitrary voltages and temperature, however this article will focus specifically
on temperature monitoring and is therefore intended as a supplement to the MAX1253/54 and MAX1153/54
data sheet, which covers all the capabilities of the devices. Please see the device data sheet for alarming
functions, device programming, specifications, etc.
Basic Diode Temperature Measurements
The forward voltage of a diode or diode-connected transistor (V
BE
) when operating at a constant current
exhibits a negative temperature co-efficient of approximately 2mV/°C which can be used to measure the
temperature at a remote diode. However, V
BE
can vary between diode manufacturers or even from device-to-
device making absolute temperature measurements unpractical for volume production since calibration is
required for each sensor. An alternate technique, which compensates for V
BE
variation uses measurements
from two known currents to calculate temperature. This measurement depends on the ratio, rather than the
absolute, and is inherently more accurate. The following equation is used in the MAX1253/1254 and
MAX1153/1154.
Temperature = (V
HIGH
- V
LOW
) × q/(n × k × ln(I
HIGH
/I
LOW
) (°K)
Where:
V
HIGH
= sensor diode voltage with high current flowing (I
HIGH
)
V
LOW
= sensor diode voltage with low current flowing (I
LOW
)
q = charge of electron = 1.602. 10
-19
Coulombs
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