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ADT7475
Enabling and Disabling THERM on Individual
Channels
THERM can be enabled/disabled for individual or
combinations of temperature channels using Bits <7:5> of
Configuration Register 5 (0x7C).
THERM Hysteresis
voltage applied across the fan and, therefore, the maximum
operating speed of the fan.
Figure 30 shows how to drive a 3-wire fan using PWM
control.
12 V 12 V
Setting Bit 0 of Configuration Register 7 (0x11) disables
THERM hysteresis.
If THERM hysteresis is enabled and THERM is disabled
(Bit 2 of Configuration Register 4, 0x7D), the THERM pin
does no t assert low when a THERM event occurs. If
THERM hysteresis is disabled and THERM is disabled
(Bit 2 of Configuration Register 4, 0x7D , and a ssuming the
appropriate pin is configured as THERM), the THERM pin
TACH
ADT7475
PWM
10 k W
10 k W
4.7 k W
3.3 V
10 k W
12 V
FAN
Q1
NDT3055L
asserts low when a THERM event occurs.
If THERM and THERM hysteresis are both enabled, the
THERM output asserts as expected.
THERM Operation in Manual Mode
In manual mode, THERM events do not cause fans to go
to full speed, unless Bit 3 of Configuration Register 6
(0x10) is set to 1.
Additionally, Bit 3 of Configuration Register 4 (0x7D)
can be used to select PWM speed on THERM event (100%
or maximum PWM).
Bit 2 in Configuration Register 4 (0x7D) can be set to
disable THERM events from affecting the fans.
Fan Drive Using PWM Control
The ADT7475 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 may need only a pullup resistor. In many cases,
Figure 30. Driving a 3-wire Fan Using an N-channel
MOSFET
Figure 30 uses a 10 k W pullup 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 ADT7475. If in doubt as to whether the fan
used 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 31 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 so that the transistor
is saturated when the fan is powered on.
the 4-wire fan PWM input has a built-in pullup resistor.
The ADT7475 PWM frequency can be set to a selection
12 V
12 V
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, so
SOT devices can be used where board space is a concern. In
TACH
ADT7475
PWM
10 k W
10 k W
TACH
4.7 k W
3.3 V
665 W
12 V
FAN
Q1
MMBT2222
desktops, fans can typically draw 250 mA to 300 mA each.
If several fans are driven 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, V GS < 3.3 V, for direct interfacing
to the PWM output 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
Figure 31. Driving a 3-wire Fan Using
an NPN Transistor
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 32 shows a typical drive circuit for 4-wire fans.
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