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ADT7462
drive outputs. If all eight fans are being used in the system,
two fans should be driven in parallel from each PWM
If the fan TACH output has a resistive pullup to V CC , it can
be connected directly to the fan input, as shown in Figure 43.
output. Figure 42 shows how to drive two fans in parallel
using the NDT3055L MOSFET. This information is
12 V
V CC
relevant for low frequency mode only (2-wire and 3-wire
fans), because the PWM and TACHs need to be
synchronized to obtain accurate fan speed measurements
using pulse stretching (see the Fan Speed Measurement with
Pulse Stretching section). In high frequency mode and when
PULLUP
4.7 k W
TYP
TACH
OUTPUT
TACH
ADT7462
FAN SPEED
COUNTER
using 4-wire fans, the TACH signal is always valid because
the fan is always powered on.
Note that because the MOSFET can handle up to 3.5 A, it
is simply a matter of connecting another fan directly in
parallel with the first. Care should be taken in designing
drive circuits with transistors and FETs to ensure that the
PWM pins are not required to source current and that they
sink less than the 8 mA maximum current specified on the
MOSFET data sheet.
3.3 V
10 k W
TYP
Figure 43. Fan with TACH Pullup to V CC
If the fan output has a resistive pullup to 12 V (or other
voltage greater than 5.0 V), the fan output can be clamped
with a Zener diode, as shown in Figure 44. The Zener diode
voltage should be chosen so that it is greater than V IH of the
TACH input but less than 5.0 V, allowing for the voltage
tolerance of the Zener diode. A value of between 3.0 V and
5.0 V is suitable.
TACH7
3.3 V
+V
+V
12 V
V CC
ADT7462
TACH3
PWM3
10 k W
TYP
TACH
3.3 V
10 k W
TYP
5V
or
12 V
FAN
Q1
NDT3055L
TACH
5V
or
12 V
FAN
PULLUP
4.7 k W
TYP
TACH
OUTPUT
ZD1*
TACH
ADT7462
FAN SPEED
COUNTER
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ? V CC
Figure 42. Interfacing Two Fans in Parallel to a PWM
Output Using a Single N-channel MOSFET
Fan Speed Measurement and Control
TACH Inputs
Pin 1, Pin 2, Pin 3, Pin 4, Pin 7, Pin 8, Pin 21, and Pin 22
are TACH inputs intended for fan speed measurement.
Signal conditioning in the ADT7462 accommodates the
slow rise and fall times typical of fan tachometer outputs.
The maximum input signal range is 0 V 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 that exceed 0 V to 5.0 V, either
resistive attenuation of the fan signal or diode clamping
must be included to keep inputs within an acceptable range.
Figure 44. Fan with TACH Pullup to Voltage > 5.0 V,
(Example 12 V) Clamped with Zener Diode
If the fan has a strong pullup (less than 1 k W ) to 12 V or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 45. Alternatively, a
resistive attenuator can be used, as shown in Figure 46. R1
and R2 should be chosen such that:
2 V t V PULLUP R2 (R PULLUP ) R1 ) R2) t 5 V
The fan inputs have an input resistance of nominally
160 k W to ground, so this should be taken into account when
calculating resistor values.
With a pullup voltage of 12 V and a pullup resistor of less
than 1 k W , suitable values for R1 and R2 would be 100 k W
and 47 k W . This gives a high input voltage of 3.83 V.
Figure 43 to Figure 46 show circuits for most common fan
TACH circuits.
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