ADT7468 Datasheet, PDF (28/81 Pages) Analog Devices – dBCool™ Remote Thermal Controller and Voltage Monitor

ADT7468
ACTIVE COOLING
DRIVING THE FAN USING PWM CONTROL
The ADT7468 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 ADT7468 PWM frequency can be set to a selection of low
frequencies or a single high PWM frequency. The low fre-
quency options are usually used for 2-wire and 3-wire fans,
while the high frequency option is usually used with 4-wire
fans.
For 2-wire or 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 and the input capacitance of the FET. Because a 10 kΩ
(or greater) resistor must be used as a PWM pull-up, an FET
with large input capacitance can cause the PWM output to
become distorted and adversely affect the fan control range.
This is a requirement only when using high frequency
PWM mode.
Typical notebook fans draw a nominal 170 mA, and so 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 requirements. The only other stipulation is that the
MOSFET should have a gate voltage drive, VGS < 3.3 V, for direct
interfacing to the PWM_OUT pin. VGS can be greater than 3.3 V
as long as the pull-up on the gate is tied to 5 V. 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 34 shows how to drive a 3-wire fan using PWM control.
12V 12V
TACH/AIN
ADT7468
PWM
10kΩ
10kΩ
4.7kΩ
3.3V
10kΩ
12V
FAN
1N4148
Q1
NDT3055L
Figure 34. Driving a 3-Wire Fan Using an N-Channel MOSFET
Figure 34 uses a 10 kΩ 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 damaging the ADT7468. If in
doubt as to whether the fan has an open-collector or totem-pole
TACH output, use one of the input signal conditioning circuits
shown in the Fan Speed Measurement section.
Figure 35 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. Ensure that the base
resistor is chosen such 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,
especially for high frequency applications.
12V 12V
TACH
ADT7468
PWM
10kΩ
10kΩ
4.7kΩ ^TACH
3.3V
665Ω
12V
FAN
1N4148
Q1
MMBT2222
Figure 35. Driving a 3-Wire Fan Using an NPN Transistor
Figure 36 shows a typical drive circuit for 4-wire fans.
TACH/AIN
ADT7468
12V 12V
10kΩ
10kΩ
TACH
4.7kΩ
3.3V
12V, 4-WIRE FAN
V_CC
TACH
PWM
2kΩ
PWM
Figure 36. Driving a 4-Wire Fan
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