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NCT7491
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accommodates the slow rise and fall times typical of fan
tachometer outputs. The maximum input signal range is 0 V
to 3.6 V, even though VCC is 3.3 V. In the event that these
inputs are supplied from fan outputs that exceed 0 V to 3.6 V,
either resistive attenuation of the fan signal or diode
clamping must be included to keep inputs within an
acceptable range. Figure 18 to Figure 20 show circuits for
the most common fan TACH outputs. If the fan TACH
output has a resistive pullup to VCC, it can be connected
directly to the fan input, as shown in Figure 18.
Figure 18. Fan with TACH Pullup to VCC
If the fan output has a resistive pullup to 12 V, or other
voltage greater than 3.6 V, the fan output can be clamped
with a zener diode, as shown in Figure 19. The zener diode
voltage should be chosen so that it is greater than VIH of the
TACH input but less than 3.6 V, allowing for the voltage
tolerance of the zener. A value of between 3.0 V and 3.6 V
is suitable.
Figure 19. Fan with TACH Pullup to Voltage > 3.6 V, for Example, 12 V Clamped with Zener Diode
If the fan has a strong pullup (less than 1 k_) to 12 V or a totempole output, a series resistor can be added to limit the zener
current, as shown in Figure 20.
Figure 20. Fan with Strong TACH Pullup to >VCC or TotemPole Output, Clamped with Zener Diode and Resistor
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