ADT7473 Datasheet, PDF (25/74 Pages) Analog Devices – dBCool Remote Thermal Monitor and Fan Controller

ADT7473
Many fans have internal pullups 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 requirements. The only other
stipulation is that the MOSFET have a gate voltage drive,
V_GS < 3.3 V, for direct interfacing to the 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 35 shows how to drive a 3−wire fan using PWM
control.
12V 12V
TACH
ADT7473/
ADT7473−1
PWM
10kΩ
10kΩ
4.7kΩ
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 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 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 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.
TACH
ADT7473/
ADT7473−1
PWM
12V 12V
10kΩ
10kΩ
4.7kΩ ^TACH
3.3V
665Ω
12V
FAN
1N4148
Q1
MMBT2222
Figure 36. 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 37 shows a typical drive circuit for 4−wire fans. As
the PWM input on 4−wire fans is usually internally pulled
up to a voltage greater than 3.6 V (the maximum voltage
allowed on the ADT7473/ADT7473−1 PWM output), the
PWM output should be clamped to 3.3 V using a Zener
diode.
TACH
ADT7473/
ADT7473−1
12V 12V
10kΩ
10kΩ
TACH
4.7kΩ
12V, 4−WIRE FAN
V_CC
TACH
PWM
PWM
3.3V
Figure 37. Driving a 4−Wire Fan
Driving Two Fans from PWM3
The ADT7473/ADT7473−1 has four TACH inputs
available for fan speed measurement, but only three PWM
drive outputs. If a fourth fan is used in the system, it should
be driven from the PWM3 output in parallel with the third
fan. Figure 38 shows how to drive two fans in parallel using
low cost NPN transistors. Figure 39 shows the equivalent
circuit using a MOSFET.
Because the MOSFET can handle up to 3.5 A, it is simply
a matter of connecting another fan directly in parallel with
the first. Care should be taken in designing drive circuits
with transistors and FETs to ensure the PWM pins are not
required to source current and that they sink less than the
8 mA maximum current specified on the data sheet.
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