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ADT7467
Figure 33 uses a 10 k W pull-up resistor for the TACH
signal. This assumes that the TACH signal is an
open-collector from the fan. In all cases, the TACH signal
from the fan must be kept below 5 V maximum to prevent
transistors. Figure 37 shows the equivalent circuit using a
MOSFET.
12 V
damaging the ADT7467. If in doubt as to whether the fan
3.3 V
3.3 V
used has an open-collector or totem pole TACH output, use
ADT7467
TACH3
TACH4
one of the input signal conditioning circuits shown in the Fan
Speed Measurement section.
Figure 34 shows a fan drive circuit using an NPN
transistor such as a general-purpose MMBT2222. Although
these devices are inexpensive, they tend to have much lower
PWM3
1 k W
2.2 k W
Q1
MMBT3904
10 W
Q2
MMBT2222
current handling capabilities and higher on resistance than
MOSFETs. When choosing a transistor, care should be taken
to ensure that it meets the fan’s current requirements.
Ensure that the base resistor is chosen such that the
10 W
Q3
MMBT2222
transistor is saturated when the fan is powered on.
Because 4-wire fans are powered continuously, the fan
speed is not switched on or off as with previous PWM
driven/powered fans. This enables it to perform better than
3-wire fans, especially for high frequency applications.
Figure 36. Interfacing Two Fans in Parallel to the
PWM3 Output Using Low Cost NPN Transistors
3.3 V
10 k W
TACHx
TACH
Figure 35 shows a typical drive circuit for 4-wire fans.
12 V 12 V
10 k W
12 V
FAN
4.7 k W
ADT7467 3.3 V
665 W
Q1
PWMx
MMBT2222
TACH4
ADT7467
TACH3
PWM3
10 k W
TYP
3.3 V
10 k W
TYP
TACH
3.3 V
10 k W
TYP
+V
5V
or
12 V
FAN
Q1
NDT3055L
TACH
+V
5V
or
12 V
FAN
Figure 34. Driving a 3-wire Fan Using
an NPN Transistor
Figure 37. Interfacing Two Fans in Parallel to the
PWM3 Output Using a Single N-channel MOSFET
TACHx
ADT7467
12 V
10 k W
10 k W
TACH
4.7 k W
3.3 V
12 V
12 V, 4-WIRE FAN
V CC
TACH
PWM
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
5 mA maximum current specified on the data sheet.
Driving up to Three Fans from PWM3
2 k W
PWMx
Figure 35. Driving a 4-wire Fan
Driving Two Fans from PWM3
The ADT7467 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 36 shows
how to drive two fans in parallel using low cost NPN
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; 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 36 and
Figure 37. The SYNC bit in Register 0x62 enables this
function.
Synchronization is not required in high frequency mode
when used with 4-wire fans.
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