Analog Devices dBCool ADT7473 Manual page 26

ADT7473
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. T
external circuitry required to drive a fan using PWM control is
extremely simp le. 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
fre quencies or a single high PWM frequency. The low
frequency options ar e usually used for 3-wir
high frequency option is usually used with 4-wire fans.
Note that care must be taken to ensure that the PWM or TACH
pins are not connected to a pull-up supply greater than 3.6 V.
Many fans have internal pull-ups connected to the TACH/
PWM pins to a supply greater than 3.6 V. Clamping or dividing
down the voltage on these pins must be done where necessary.
Clamping these pins with a Zener diode can also help prevent
back-EMF related noise from being coupled into the system.
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
no tebook 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 seve
fans in parallel from a single PWM output or drive larger s
fans, the MOSFET must handle the higher current require-
ments. The on ly other stipulation is that the MOSFET have a
gate voltage dri ve, V < 3.3 V, for direct interfacing to the
GS
PWM output. 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 fa
Figure 32 shows how to drive a 3-wire fan using PWM contro
10kΩ
TACH
4.7kΩ
ADT7473
10kΩ
PWM
Figure 32. Driving a 3-Wire Fa
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
e fans, while the
12 V 12V
10kΩ
12V
1N4148
FAN
3.3V
Q1
NDT3055L
n U sing an N-Channel MOSFET
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
such as a general-purpose MMBT2222. While these devices are
he
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.
Ensure that the base resistor is chosen so that the transistor is
saturated when the fan is powered on.
ADT7473
Figure 33. Driving a 3-Wire Fan Using an NPN Transistor
Beca use 4-wire fans are powered continuously, the fa
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.
ral Figure 34 shows
erver PWM input on 4-wire fans is usually internally pulled up to a
voltage greater than 3.6 V (the maximum voltage allowed on
ADT7473 PWM output), the PWM output should be c
to 3.3 V using a Zener diode.
n.
l.
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 used in the system, it should be driven from the PWM3
output in parallel with the third fan. Figure 35 shows h
drive two fans in parallel using low cost NPN transistors.
Figure 36 shows the equivalent
Because the MOSFET can handle up to 3.5 A, it is simply a
matter of connecting another fan directly in parallel with the
Rev. A | Page 26 of 76
12V 12V
10kΩ
10kΩ
TACH
TACH
4.7kΩ
3.3V
665Ω
Q1
PWM
MMBT2222
a typical drive circuit for 4-wire fans. As the
12V 12V
12V, 4-WIRE FAN
10k Ω
V
10k Ω
TACH TACH
TACH
4.7k Ω PWM
PWM
3.3V
Figure 34. Driving a 4-Wire Fan
circuit using a MOSFET.
-
NPN transistor
12V
1N4148
FAN
n speed is
the
lamped
CC
ow to