Fan Drive Using Pwm Control - Analog Devices ADT7473 Manual

Configuring the THERM Pin as Bidirectional
In addition to monitoring THERM as an input, the ADT7473/
ADT7473-1 can optionally drive THERM low as an output.
When PROCHOT is bidirectional, THERM can be used to
throttle the processor by asserting PROCHOT . The user can
preprogram system-critical thermal limits. If the temperature
exceeds a thermal limit by 0.25°C, THERM asserts low. If the
temperature is still above the thermal limit on the next monitor-
ing cycle, THERM stays low. THERM remains asserted low
until the temperature is equal to or below the thermal limit.
Because the temperature for that channel is measured only once
for every monitoring cycle after THERM asserts, it is guaran-
teed to remain low for at least one monitoring cycle.
The THERM pin can be configured to assert low, if the
Remote 1, local, or Remote 2 THERM temperature limits are
exceeded by 0.25°C. The THERM temperature limit registers
are at Register 0x6A, Register 0x6B, and Register 0x6C, respec-
tively. Setting Bit 5, Bit 6, and Bit 7 of Configuration Register 5
(0x7C) makes THERM bidirectional for the Remote 1, local,
and Remote 2 temperature channels, respectively. Figure 36
shows how the THERM pin asserts low as an output in the
event of a critical over temperature.
An alternative method of disabling THERM is to program the
THERM temperature limit to –64°C or less in Offset 64 mode,
or −128°C or less in twos complement mode; that is, for
THERM temperature limit values less than –63°C or –128°C,
respectively, THERM is disabled. THERM can also be disabled
by setting Bit 1 of Configuration Register 3 (0x78) to 0.
THERM LIMIT
0.25°C
THERM LIMIT
TEMP
THERM
MONITORING
CYCLE
Figure 36. Asserting THERM as an Output,
Based on Tripping THERM Limits

FAN DRIVE USING PWM CONTROL

The ADT7473/ADT7473-1 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 ADT7473/ADT7473-1 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.
Note that care must be taken to ensure that the PWM or TACH
pins are not connected to a pull-up supply greater than 3.6 V.
Many fans have internal pull-ups connected to the TACH/
PWM pins to a supply greater than 3.6 V. Clamping or dividing
down the voltage on these pins must be done where necessary.
Clamping these pins with a Zener diode can also help prevent
back-EMF related noise from being coupled into the system.
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; therefore, 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 require-
ments. The only other stipulation is that the MOSFET have a
gate voltage drive, V
PWM output. 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 37 shows how to drive a 3-wire fan using PWM control.
ADT7473/
ADT7473-1
Figure 37. Driving a 3-Wire Fan Using an N-Channel MOSFET
Figure 37 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 ADT7473/
ADT7473-1. If uncertain 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 38 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.
Rev. C | Page 25 of 72
ADT7473/ADT7473-1
< 3.3 V, for direct interfacing to the
GS
12V 12V
10kΩ
10kΩ
TACH
4.7kΩ
3.3V
10kΩ
Q1
PWM
NDT3055L
12V
1N4148
FAN