Application Hints (Continued)
5.0
VREF AND VTEMP CAPACTIVE LOADING
DS012893-19
FIGURE 4. Loading of VREF and VTEMP
The LM56 VREF and VTEMP outputs handle capacitive load-
ing well. Without any special precautions, these outputs can
drive any capacitive load as shown in Figure 4 .
The circuit shown inFigure 5 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:
6.0 NOISY ENVIRONMENTS
Over the specified temperature range the LM56 VTEMPout-
put has a maximum output impedance of 1500Ω. In an ex-
tremely noisy environment it may be necessary to add some
filtering to minimize noise pickup. It is recommended that 0.1
µF be added from V+ to GND to bypass the power supply
voltage, as shown in Figure 4 . In a noisy environment it may
be necessary to add a capacitor from the VTEMP output to
ground. A 1 µF output capacitor with the 1500Ω output im-
pedance will form a 106 Hz lowpass filter. Since the thermal
time constant of the VTEMP output is much slower than the
9.4 ms time constant formed by the RC, the overall response
time of the VTEMP output will not be significantly affected. For
much larger capacitors this additional time lag will increase
the overall response time of the LM56.
where IB = 300 nA (the maximum specified error).
Similarly, bias current affect on VT2 can be defined by:
7.0 APPLICATIONS CIRCUITS
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
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.
DS012893-20
FIGURE 5. Reducing Errors Caused by Bias Current
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