ADT7470 Analog Devices, Inc., ADT7470 Datasheet - Page 19

• Manufacturer Part Number: ADT7470
• Description: Temperature Sensor Hub And Fan Controller
• Manufacturer: Analog Devices, Inc.

FAN DRIVE USING PWM CONTROL
The ADT7470 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. Two
main control schemes are used: low frequency and high fre-
quency PWM. For low frequency, low-side drive, the external
circuitry required to drive a fan using PWM control is extremely
simple. A single NMOS FET 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 the user needs to drive several
fans in parallel from a single PWM output or drive larger server
fans, the MOSFET needs to 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_OUT pin of the TSM devices. VGS of the chosen
MOSFET can be greater than 3.3 V as long as the pull-up on its
gate is tied to 5 V. The MOSFET should also have a low on
resistance to ensure that there is not significant voltage drop
across the FET. This would reduce the voltage applied across the
fan and, therefore, the maximum operating speed of the fan.
Figure 23 shows how a 3-wire fan can be driven using low
frequency PWM control where the control method is low-side,
low frequency switching.
Figure 23 shows the ideal interface when interfacing a tach
signal from a 12 V fan (or greater voltage) to a 5 V (or less)
logic device. In all cases, the tach signal from the fan must be
kept below 5 V maximum to prevent damage to the ADT7470.
The three resistors in Figure 23 ensure that the tach voltage is
kept within safe levels for typical desktop and notebook
systems.
ADT7470
Figure 23. Driving a 3-Wire Fan Using an N-Channel MOSFET
TACH/AIN
PWM
4.7kΩ
10kΩ
3.3V
10kΩ
TACH
12V
10kΩ
12V
NDT3055L
Q1
12V
FAN
1N4148
Rev. B | Page 19 of 40
Figure 24 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. This is the only major
difference between a MOSFET and NPN transistor fan driver
circuit.
When using transistors, ensure that the base resistor is chosen
such that the transistor is fully saturated when the fan is
powered on. Otherwise, there are power inefficiencies in the
implementation.
High Frequency vs. Low Frequency
One of the important features of fan controllers is the PWM
drive frequency. Most fans are driven asynchronously at low
frequency (30 Hz to 100 Hz). Increasingly, the devices drive
fans at >20 kHz. These controllers are meant to drive 4-wire
fans with PWM control built-in internal to the fan in Figure 25.
The ADT7470 supports high frequency PWM (>20 kHz)
as well as 1.4 kHz and other low frequency PWM. This allows
the user to drive 3-wire or 4-wire fans.
ADT7470
ADT7470
Figure 24. Driving a 3-Wire Fan Using an NPN Transistor
TACH/AIN
TACH
PWM
PWM
Figure 25. Driving a 4-Wire Fan
4.7kΩ
4.7kΩ
10kΩ
10kΩ
3.3V
3.3V
470Ω
10kΩ
TACH
TACH
12V
PWM_IN
12V
10kΩ
10kΩ
12V
V
GND
MMBT2222
12V
FAN
Q1
ADT7470
1N4148
1N4148