LM56 datasheet(12/21 Pages) TI

12 / 21 page

LM56

SNIS120G – APRIL 2000 – REVISED FEBRUARY 2013

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The circuit shown in Figure 17 will reduce the effective bias current error for VT2 as discussed in Section 3.0 to

be equivalent to the error term of VT1. For this circuit the effect of the bias current on the first trip point can be

defined by the following equations:

(3)

where IB = 300 nA (the maximum specified error).

Similarly, bias current affect on VT2 can be defined by:

(4)

where IB = 300 nA (the maximum specified error).

The current shown in Figure 18 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 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.

Figure 18. Audio Power Amplifier Overtemperature Detector

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LM56 Datasheet(PDF) 12 Page - Texas Instruments