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ADT7473
12 V
12 V
Because the MOSFET can handle up to 3.5 A, it is simply
TACH
ADT7473/
10 k W
4.7 k W
10 k W
TACH
12 V, 4-WIRE FAN
V CC
TACH
PWM
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 the PWM pins are not
required to source current and that they sink less than the
8 mA maximum current specified on the data sheet.
ADT7473 ? 1
PWM
3.3 V
Figure 37. Driving a 4-wire Fan
Driving Two Fans from PWM3
The ADT7473/ADT7473 ? 1 has four TACH inputs
available for fan speed measurement, but only three PWM
drive outputs. If a fourth fan is used in the system, it should
be driven from the PWM3 output in parallel with the third
fan. Figure 38 shows how to drive two fans in parallel using
low cost NPN transistors. Figure 39 shows the equivalent
circuit using a MOSFET.
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 programmed 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 38 and
Figure 39. The SYNC bit in Register 0x62 enables this
function.
Synchronization is not required in high frequency mode
when used with 4-wire fans.
Table 33. SYNC: ENHANCED ACOUSTICS
REGISTER 1 (REG. 0X62)
12 V
Bit
Mnemonic
Description
3.3 V
ADT7473/
ADT7373 ? 1
PWM3
3.3 V
1 k W
2.2 k W
TACH3
3.3 V
Q1
MMBT3904
Q2
TACH4
3.3 V
<4> SYNC 1 Synchronizes TACH2, TACH3, and
TACH4 to PWM3.
TACH Inputs
Pin 4, Pin 6, Pin 7, and Pin 9 (when configured as TACH
inputs) are open-drain TACH inputs intended for fan speed
measurement.
10 k W
10 k W
MMBT2222
Q3
MMBT2222
Signal conditioning in the ADT7473/ADT7473 ? 1
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
Figure 38. Interfacing Two Fans in Parallel to the
PWM3 Output Using Low Cost NPN Transistors
3.3 V
10 k W
TYP
of the fan signal or diode clamping must be included to keep
inputs within an acceptable range.
Figure 40 to Figure 43 show circuits for most common fan
TACH outputs.
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 40.
TACH4
3.3 V
3.3 V
+V
+V
12 V
V CC
ADT7473/
ADT7473 ? 1
TACH3
10 k W
TYP
3.3 V
TACH
3.3 V
5V
or
12 V
FAN
TACH
5V
or
12 V
FAN
PULLUP
4.7 k W
TYP
TACH
OUTPUT
TACH
FAN SPEED
COUNTER
PWM3
10 k W
TYP
Q1
NDT3055L
ADT7473/
ADT7473 ? 1
Figure 39. Interfacing Two Fans in Parallel to the
PWM3 Output Using a Single N-channel MOSFET
http://onsemi.com
26
Figure 40. Fan with TACH Pullup to V CC
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