ADT7476 Datasheet, PDF (26/67 Pages) Analog Devices – dBCool Remote Thermal Controller and Voltage Monitor

ADT7476
does not assert low when a THERM event occurs. If
THERM hysteresis is disabled and THERM is disabled
(Bit 2 of Configuration Register 4, 0x7D) and assuming the
appropriate pin is configured as THERM), the THERM pin
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 the PWM speed on a 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 ADT7476 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 pullup resistor. In many cases,
the 4−wire fan PWM input has a built−in, pullup resistor.
The ADT7476 PWM frequency can be set to a selection
of low frequencies or a single high PWM frequency. The
low frequency options are 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 and the input capacitance of the FET. Because a
10 kW (or greater) resistor must be used as a PWM pullup,
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, so SOT
devices can be used where board space is a concern. In
desktops, fans 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,
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 a 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
35 shows how to drive a 3−wire fan using PWM control.
TACH
ADT7476
PWM
12V 12V
10kΩ
10kΩ
4.7kΩ ^TACH
12V
FAN
1N4148
3.3V
10kΩ
Q1
NDT3055L
Figure 35. Driving a 3−Wire Fan Using an N−Channel
MOSFET
Figure 35 uses a 10 kW 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 5.5 V maximum to prevent
damaging the ADT7476.
Figure 36 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.
Because the fan drive circuitry in 4−wire fans is not
switched on or off, as with previous PWM driven/powered
fans, the internal drive circuit is always on and uses the
PWM input as a signal instead of a power supply. This
enables the internal fan drive circuit to perform better than
3−wire fans, especially for high frequency applications.
12V 12V
TACH
ADT7476
10kΩ
10kΩ
4.7kΩ ^TACH
3.3V
12V
FAN
1N4148
PWM
470Ω
Q1
MMBT2222
Figure 36. Driving a 3−Wire Fan Using an
NPN Transistor
Figure 37 shows a typical drive circuit for 4−wire fans.
TACH
ADT7476
12V 12V
10kΩ
10kΩ
TACH
4.7kΩ
12V, 4−WIRE FAN
V_CC
TACH
PWM
3.3V
2kΩ
PWM
Figure 37. Driving a 4−Wire Fan
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