ADT7462ACPZ-R7 datasheet(27/81 Pages) ONSEMI

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ADT7462

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Figure 39. Driving a 2−Wire Fan

ADT7462

PWM

TACH

5.0 V or

12 V FAN

NDT3055L

3.3 V

10 kΩ

TYPICAL

1N4148

0.01 mF

RSENSE

2 Ω

TYPICAL

Using the ADT7462 with 3−Wire Fans

Figure 40 shows the most typical circuit used with a

3−wire fan.

Figure 40. Driving a 3−Wire Fan

ADT7462

TACH/AIN

PWM

12 V

FAN

NDT3055L

3.3 V

12 V

1N4148

10 kΩ

4.7 kΩ

The external circuitry required is very simple. A

MOSFET, such as the NDT3055L, is used as the pass device.

The specifications of the MOSFET depend on the maximum

current required by the fan being driven. A typical PC fan

can draw a nominal current ranging from a few hundred

milliamps to over an amp of current. Depending on the

current rating of the fan, a SOT device can be used where

board space is a concern. If several fans in parallel are driven

from a single PWM output or if larger server fans are driven,

the MOSFET must handle the higher current requirements.

The only other stipulation is that the MOSFET should have

a gate voltage drive, VGS < 3.3 V, for direct interfacing to the

PWM pins. VGS can be greater than 3.3 V as long as the

pullup on the gate is tied to 5.0 V. The MOSFET should also

have a low on resistance to ensure that there is not a

significant voltage drop across the FET, which would reduce

the voltage applied across the fan and reduce the full speed

of the fan.

Figure 40 uses a 10 k

W 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 5.0 V maximum to prevent

damaging the ADT7462. 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.

Driving a 3−wire fan with a PWM signal makes the fan

speed measurement more difficult because the TACH signal

is chopped by the PWM drive signal. Pulse stretching is

required in this case to make accurate fan speed

measurements. For more information, see the Fan Speed

Measurement section.

Using the ADT7462 with 4−Wire Fans

Figure 41 shows the most typical circuit used with 4−wire

fans.

Figure 41. Driving a 4−Wire Fan

ADT7462

TACH

PWM

12 V, 4−WIRE FAN

3.3 V or 5.0 V

12 V

VCC

TACH

PWM

2 kΩ

10 kΩ

4.7 kΩ

12 V

Because the electronics in a 4−wire fan are powered

continuously, unlike previous PWM driven/powered fans,

4−wire fans tend to perform better than 3−wire fans,

especially for high frequency applications. 4−wire frames

also eliminate the requirement for pulse stretching, because

the TACH signal is always available.

Driving Two Fans from Each PWM

Note that the ADT7462 has up to eight TACH inputs

available for fan speed measurement, but only four PWM

drive outputs. If all eight fans are being used in the system,

two fans should be driven in parallel from each PWM

output. Figure 42 shows how to drive two fans in parallel

using the NDT3055L MOSFET. This information is

relevant for low frequency mode only (2−wire and 3−wire

fans), because the PWM and TACHs need to be

synchronized to obtain accurate fan speed measurements

using pulse stretching (see the Fan Speed Measurement with

Pulse Stretching section). In high frequency mode and when

using 4−wire fans, the TACH signal is always valid because

the fan is always powered on.

Note that 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 that the

PWM pins are not required to source current and that they

sink less than the 8 mA maximum current specified on the

MOSFET data sheet.

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