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ADT7473
Driving up to Three Fans from PWM3
TACH measurements for fans are synchronized to particular
PWM channels, for example, TACH1 is synchronized to
PWM1. TACH3 and TACH4 are both synchronized to PWM3,
so PWM3 can drive two fans. Alternatively, PWM3 can be pro-
grammed to synchronize TACH2, TACH3, and TACH4 to the
PWM3 output. This allows PWM3 to drive two or three fans. In
this case, the drive circuitry looks the same, as shown in
Figure 35 and Figure 36. The SYNC bit in Register 0x62 enables
this function.
Synchronization is not required in high frequency mode when
used with 4-wire fans.
<4> (SYNC) Enhance Acoustics Register 1 (Reg. 0x62)
SYNC = 1 , synchronizes TACH2, TACH3, and TACH4 to
PWM3.
TACH Inputs
Pins 4, 6, 7, and 9 (when configured as TACH inputs) are open-
drain TACH inputs intended for fan speed measurement.
ADT7473
3.3V
1kΩ
PWM3
2.2kΩ
Figure 35. Interfacing Two Fans in Parallel to the PWM3 Output Using Low Cost NPN Transistors
3.3V
10kΩ
TYPICAL
TACH4
3.3V
ADT7473
10kΩ
TYPICAL
TACH3
3.3V
10kΩ
TYPICAL
PWM3
Figure 36. Interfacing Two Fans in Parallel to the PWM3 Output Using a Single N-Channel MOSFET
Signal conditioning in the ADT7473 accommodates the slow
rise and fall times typical of fan tachometer outputs. The
maximum input signal range is 0 V to 3.6 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 37 to Figure 40 show circuits for most common fan
TACH outputs.
If the fan TACH output has a resistive pull-up to V
connected directly to the fan input, as shown in Figure 37.
If the fan output has a resistive pull-up 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 38. The Zener diode voltage should
be chosen so that it is greater than V
less than 3.6 V, allowing for the voltage tolerance of the Zener. A
value of between 3 V and 3.6 V is suitable.
3.3V
TACH3
3.3V
Q1
MMBT3904
10Ω
Q2
MMBT2222
10Ω
Q3
MMBT2222
3.3V
+V
1N4148
TACH
5V OR
12V FAN
TACH
3.3V
Q1
NDT3055L
Rev. 0 | Page 26 of 76
of the TACH input but
IH
12V
TACH4
3.3V
+V
5V OR
12V FAN
, it can be
CC
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