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FAN DRIVE USING PWM CONTROL
The ADT7473 uses pulse-width modulation (PWM) to control
fan speed. This relies on varying the duty cycle (or on/off ratio)
of a square wave applied to the fan to vary the fan speed. The
external circuitry required to drive a fan using PWM control is
extremely simple. For 4-wire fans, the PWM drive might need
only a pull-up resistor. In many cases, the 4-wire fan PWM
input has a built-in pull-up resistor.
The ADT7473 PWM frequency can be set to a selection of low
frequencies or a single high PWM frequency. The low
frequency options are usually used for 3-wire fans, while the
high frequency option is usually used with 4-wire fans.
For 3-wire fans, a single N-channel MOSFET is the only drive
device required. The specifications of the MOSFET depend on
the maximum current required by the fan being driven. Typical
notebook fans draw a nominal 170 mA; therefore, SOT devices
can be used where board space is a concern. In desktops, fans
can typically draw 250 mA to 300 mA each. If you drive several
fans in parallel from a single PWM output or drive larger server
fans, the MOSFET must handle the higher current require-
ments. The only other stipulation is that the MOSFET should
have a gate voltage drive, V
GS
the PWM_OUT pin. The MOSFET should also have a low on
resistance to ensure that there is not significant voltage drop
across the FET, which would reduce the voltage applied across
the fan and, therefore, the maximum operating speed of the fan.
Figure 32 shows how to drive a 3-wire fan using PWM control.
10kΩ
TACH/AIN
4.7kΩ
ADT7473
10kΩ
PWM
Figure 32. Driving a 3-Wire Fan Using an N-Channel MOSFET
Figure 32 uses a 10 k Ω 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 3.6 V maximum to prevent damaging the ADT7473. If
uncertain as to whether the fan used has an open-collector or
totem pole TACH output, use one of the input signal condition-
ing circuits shown in the Fan Speed Measurement section.
Figure 33 shows a fan drive circuit using an NPN transistor
such as a general-purpose MMBT2222. While these devices are
inexpensive, they tend to have much lower 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.
< 3.3 V, for direct interfacing to
12V
12V
10kΩ
12V
1N4148
FAN
3.3V
Q1
NDT3055L
Ensure that the base resistor is chosen so that the 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, espe-
cially for high frequency applications.
Figure 34 shows a typical drive circuit for 4-wire fans. As the
PWM input on 4-wire fans is usually internally pulled up to a
voltage greater than 3.6 V (the max voltage allowed on the
ADT7473 PWM output), the PWM output should be clamped
to 3.3 V using a Zener diode.
ADT7473
Figure 33. Driving a 3-Wire Fan Using an NPN Transistor
ADT7473
Driving Two Fans from PWM3
The ADT7473 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 35 shows
how to drive two fans in parallel using low cost NPN
transistors. Figure 36 shows the equivalent circuit using a
MOSFET.
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 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.
Rev. 0 | Page 25 of 76
12V
12V
10kΩ
10kΩ
TACH
TACH
4.7kΩ
3.3V
665Ω
Q1
PWM
MMBT2222
12V 12V
12V, 4-WIRE FAN
10kΩ
V
10kΩ
TACH
TACH
TACH
4.7kΩ
PWM
PWM
3.3V
Figure 34. Driving a 4-Wire Fan
ADT7473
12V
1N4148
FAN
CC
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