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 tem-
perature sensor is mounted on a PC board that is bolted to
the heat sink near the power amplifier. To ensure that the
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 approximately 75°C.
where IB = 300 nA (the maximum specified error).
Similarly, bias current affect on VT2 can be defined by:
ꢁ
where IB = 300 nA (the maximum specified error).
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FIGURE 6. Audio Power Amplifier Overtemperature Detector
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