ADT7476 Datasheet, PDF (31/72 Pages) Analog Devices – dBCool Remote Thermal Controller and Voltage Monitor

ADT7476
Enabling and Disabling THERM on individual Channels
THERM can be enabled/disabled for individual or combina-
tions of temperature channels using Bits [7:5] of Configuration
Register 5 (0x7C).
THERM Hysteresis
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 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 config-
ured 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 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 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 pull-up resistor. In many cases, the 4-wire fan PWM
input has a built-in, pull-up 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 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, 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 require-
ments. The only other stipulation is that the MOSFET should
have a gate voltage drive, VGS < 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.
12V 12V
TACH
ADT7476
PWM
10kΩ
10kΩ
4.7kΩ ^TACH
3.3V
10kΩ
12V
FAN
1N4148
Q1
NDT3055L
Figure 35. Driving a 3-Wire Fan Using an N-Channel MOSFET
Figure 35 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 3.6 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 han-
dling 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 the transistor is saturated when the
fan is powered on.
Because in 4-wire fans the fan drive circuitry 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
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