sensing element is at the same temperature as the heat sink,
the sensor’s leads are mounted to pads that have feed
throughs to the back side of the PC board. Since the LM56 is
sensing the temperature of the actual PC board the back
side of the PC board also has large ground plane to help
conduct the heat to the device. The comparator’s output
goes low if the heat sink temperature rises above a threshold
set by R1, R2, and the voltage reference. This fault detection
output from the comparator now can be used to turn on a
cooling fan. The circuit as shown in design to turn the fan on
when heat sink temperature exceeds about 80˚C, and to turn
the fan off when the heat sink temperature falls below ap-
proximately 75˚C.
Application Hints (Continued)
where IB = 300 nA (the maximum specified error).
The current shown in Figure 6 is a simple overtemperature
detector for power devices. In this example, an audio power
amplifier IC is bolted to a heat sink and an LM56 Celsius
temperature sensor is mounted on a PC board that is bolted
to the heat sink near the power amplifier. To ensure that the
01289321
FIGURE 6. Audio Power Amplifier Overtemperature Detector
01289322
FIGURE 7. Simple Thermostat
11
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