ADT7470 datasheet(21/41 Pages) AD

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ADT7470

Data Sheet

Rev. E | Page 20 of 40

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.

Configuration Register 1 Bit[6], at address 0x40, configures the

fan drive for high or low frequency operation. If this bit is set to

0, which is the default, high frequency fan drive is selected. If

this bit is set to 1, low frequency fan drive is selected. All four

PWM outputs on the ADT7470 have the same drive frequency.

HIGH FREQUENCY FAN DRIVE

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 greater than 20 kHz. These controllers are meant to drive

4-wire fans with PWM control built-in internal to the fan in

Figure 17. The ADT7470 supports high frequency PWM (great

than 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.

If using 3-wire fans this mode, care should be taken to ensure

that incomplete tach information does not occur at low PWM

duty cycles, or short PWM pulse widths.

PWM

3.3V

10k

Ω

TACH

ADT7470

12V

10k

Ω

TACH

10k

Ω

4.7k

Ω

1N4148

GND

PWM_IN

Figure 17. Driving a 4-Wire Fan

LOW FREQUENCY FAN DRIVE

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 requirements. The

only other stipulation is that the MOSFET should have a gate

voltage drive, VGS, less than 3.3 V, for direct interfacing to the

PWM pin of the ADT7470. 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 18 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 18 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 18 ensure that the tach voltage

is kept within safe levels for typical desktop and notebook

systems.

12V

NDT3055L

PWM

3.3V

10k

Ω

12V

FAN

TACH/AIN

ADT7470

12V

10k

Ω

TACH

10k

Ω

4.7k

Ω

1N4148

Figure 18. Driving a 3-Wire Fan Using an N-Channel MOSFET

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