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ADM1034
http://onsemi.com
22
measurements are the main benefits. The fan is allowed to
run reliably at speeds as low as 30 percent of the full
capability.
The drive signal applied to the fan is synchronized with
the TACH signal. The ADM1034 switches on the drive
signal and waits for a transition on the TACH signal. When
a transition takes place on the TACH signal, the PWM drive
is switched off for a period of time called t
off
. The drive
signal is then switched on again. The toff time is varied in
order to vary the fan speed. If the fan is running too fast, the
toff time is increased. If the fan is running too slow, the toff
time is decreased.
Since the drive signal is synchronized with the TACH
signal, the frequency with which the fan is driven depends
on the current speed of the fan and the number of poles in it.
Figure 34 shows how the synchronous speed drive signal
works. The ideal TACH signal is the TACH signal that
would be output from the fan if power were applied
100 percent of the time. It is representative of the actual
speed of the fan. The actual TACH signal is the signal the
user would see on the TACH output from the fan if the user
were to put a scope on it. In effect, the actual TACH signal
is the ideal TACH signal chopped with the drive signal.
Figure 34. Drive Signal by Using Synchronous Control
ACTUAL TACH
DRIVE
IDEAL TACH
POLE TRANSITION POINTS
t
POLE
t
OFF
DASH = TACH FLOATS HIGH BY PULL-UP RESISTOR
SOLID = TRUE TACH WHEN FAN IS POWERED
Fan Inputs
Pin 2 and Pin 4 are TACH inputs intended for fan speed
measurement. These inputs are open-drain.
Signal conditioning on the ADM1034 accommodates the
slow rise and fall time of typical tachometer outputs. The
maximum input signal range is from 0V to 5.0 V, even when
V
CC
 is less than 5.0 V. In the event that these inputs are
supplied from fan outputs exceeding 0 V to 5.0 V, either
resistive attenuation of the fan signal or diode clamping
must be used to keep the fan inputs within an acceptable
range.
Figure 35 to Figure 38 show examples of possible fan
input circuits. If the fan TACH has a resistive pullup to V
CC
,
it can be connected directly to the fan output.
Figure 35. Fan with TACH Pullup to +V
CC
5 V or 12 V
FAN
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH X
PULLUP
4.7 kW
TYP
ADM1034
DRIVE X
V
CC
100 kW
TYP
If the fan output has a resistive pullup to 12 V (or another
voltage greater than 5.0 V), the fan output can be clamped
with a Zener diode, as shown in Figure 36. The Zener
voltage should be chosen so that it is greater than V
IH
 but less
than 5.0 V. Allowing for the voltage tolerance of the Zener,
a value of between 3.0 V and 5.0 V is suitable.
Figure 36. Fan with TACH Pullup to Voltage > 5.0 V,
Clamped with Zener Diode
5 V or 12 V
FAN
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH X
PULLUP
4.7 kW
TYP
ADM1034
DRIVE X
V
CC
100 kW
TYP
ZD1*
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ?V
CC
If the fan has a strong pullup (less than 1 kW to +12 V) or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 37. Alternatively, a
resistive attenuator may be used, as shown in Figure 38.
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