ADT7467ARQZ-R7 datasheet(27/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 27 of 77 | www.onsemi.com

ACTIVE COOLING

DRIVING THE FAN USING PWM CONTROL

The ADT7467 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 ADT7467 PWM frequency can be set to a selection of

low frequencies or a single high PWM frequency. The low fre-

quency options are usually used for 2-wire and 3-wire fans,

and the high frequency option is usually used for 4-wire fans.

For 2-wire or 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 of VGS < 3.3 V for direct

interfacing to the PWMx pin. VGS can be greater than 3.3 V as

long as the pull-up on the 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, which would reduce the

voltage applied across the fan and, therefore, the maximum

operating speed of the fan.

Figure 34 shows how to drive a 3-wire fan using PWM control.

ADT7467

TACHx

PWMx

12V

FAN

NDT3055L

3.3V

12V

10kΩ

4.7kΩ

10kΩ

1N4148

Figure 34. Driving a 3-Wire Fan Using an N-Channel MOSFET

Figure 34 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 5 V maximum to prevent damaging the ADT7467. 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 35 shows a fan drive circuit using an NPN transistor

such as a general-purpose MMBT2222. Although 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 such that the transistor is

saturated when the fan is powered on.

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, espe-

cially for high frequency applications. Figure 36 shows a typical

drive circuit for 4-wire fans.

ADT7467

TACHx

TACH

PWMx

12V

FAN

MMBT2222

3.3V

12V

665Ω

4.7kΩ

10kΩ

1N4148

Figure 35. Driving a 3-Wire Fan Using an NPN Transistor

ADT7467

TACHx

PWMx

12V, 4-WIRE FAN

3.3V

12V 12V

2kΩ

4.7kΩ

10kΩ

VCC

TACH

PWM

Figure 36. Driving a 4-Wire Fan

Driving Two Fans from PWM3

The ADT7467 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 37 shows how to

drive two fans in parallel using low cost NPN transistors.

Figure 38 shows the equivalent circuit using a MOSFET.

ADT7467ARQZ-R7 Datasheet(PDF) 27 Page - ON Semiconductor

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