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ADT7468
Driving Two Fans from PWM3
The ADT7468 has four TACH inputs available for fan speed
measurement, but only three PWM drive outputs. If a fourth
fan is being used in the system, it should be driven from the
PWM3 output in parallel with the third fan. Figure 37 shows
how to drive two fans in parallel using low cost NPN
transistors. Figure 38 shows the equivalent circuit using a
MOSFET.
Because the MOSFET can handle up to 3.5 A, a second fan can
be connected 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 5 mA maximum current
specified in the data sheet.
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,
and therefore 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 is as shown in
Figure 37 and Figure 38. 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.
12V
ADT7468
PWM3
3.3V
1k ?
2.2k ?
3.3V
TACH3
Q1
MMBT3904
10 ?
Q2
TACH4
MMBT2222
10 ?
Q3
MMBT2222
Figure 37. Interfacing Two Fans in Parallel to the PWM3 Output Using Low Cost NPN Transistors
3.3V
10k ?
TYPICAL
TACH4
ADT7468
3.3V
10k ?
+V
+V
TYPICAL
TACH3
3.3V
TACH
5V OR
12V FAN
1N4148
TACH
5V OR
12V FAN
10k ?
TYPICAL
PWM3
Q1
NDT3055L
Figure 38. Interfacing Two Fans in Parallel to the PWM3 Output Using a Single N-Channel MOSFET
Rev. 3 | Page 29 of 81 | www.onsemi.com
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