ADT7475 Analog Devices, Inc., ADT7475 Datasheet - Page 26

• Manufacturer Part Number: ADT7475
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7475
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 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 pull-up resistor. In many cases, the 4-wire fan PWM
input has a built-in pull-up resistor.
The ADT7475 PWM frequency can be set to a selection 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 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.
Rev. B | Page 26 of 68
The only other stipulation is that the MOSFET should have a
gate voltage drive, V
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 voltage applied across
the fan and, therefore, the maximum operating speed of the fan.
Figure 31 shows how to drive a 3-wire fan using PWM control.
Figure 31 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 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 32 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 that the transistor is
saturated when the fan is powered on.
Figure 31. Driving a 3-Wire Fan Using an N-Channel MOSFET
Figure 32. Driving a 3-Wire Fan Using an NPN Transistor
ADT7475
ADT7475
TACH
TACH
PWM
PWM
GS
< 3.3 V, for direct interfacing to the
4.7kΩ
10kΩ
4.7kΩ
665Ω
10kΩ
10kΩ
3.3V
3.3V
10kΩ
10kΩ
TACH
12V
12V
12V
12V
Q1
NDT3055L
Q1
MMBT2222
12V
FAN
12V
FAN
1N4148
1N4148