ADT7475 datasheet(27/68 Pages) AD

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ADT7475

Rev. A | Page 27 of 68

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 33 shows a typical

drive circuit for 4-wire fans.

ADT7475

TACH

PWM

12V, 4-WIRE FAN

3.3V

V 12V

2kΩ

4.7kΩ

10kΩ

VCC

TACH

o Fans from PWM3

The ADT7475 has four TACH inputs available for fan speed

measurement, but only three PWM drive outputs. If a fourth

fan is being used in the system, it should be driven from the

PWM3 output in parallel with the third fan. Figure 34 shows

how to drive two fans in parallel using low cost NPN

transistors. Figure 35 shows the equivalent circuit using a

MOSFET.

TACH

PWM

Figure 33. Driving a 4-Wire Fan

Driving Tw

ADT7475

PWM3

3.3V

12V

MMBT3904

MMBT2222

10Ω

2.2kΩ

10Ω

1kΩ

TACH3

TACH4

Low

Figure 34. Interfacing Two Fans in Parallel to the PWM3 Output Using

Cost NPN Transistors

ADT7475

PWM3

TACH3

TACH4

3.3V

TACH

NDT3055L

1N4148

5V OR

12V FAN

5V OR

12V FAN

10kΩ

TYPICAL

10kΩ

TYPICAL

10kΩ

TYPICAL

rfacing Two Fans in Parallel to the PWM3 Output Using a

y a

the

hould be taken in designing drive circuits with

transistors and FETs to ensure that the PWM pins are not

rce current and that they sink less than the

TACH measurements for fans are synchronized to particular

wn in

bles

ire fans.

Bit 4 (SYNC) Enhance Acoustics Register 1 (0x62)

SYNC = 1, synchronizes TACH2, TACH3, and TACH4

to PWM3.

TACH Inputs

Pin 4, Pin 6, Pin 7, and Pin 9, when configured as TACH

inputs, are open-drain TACH inputs intended for fan speed

measurement.

Signal conditioning in the ADT7475 accommodates the slow

rise and fall times typical of fan tachometer outputs. The maxi-

mum input signal range is 0 V to 3.6 V. In the event these inputs

are supplied from fan outputs that exceed 0 V to 3.6 V, either

resistive attenuation of the fan signal or diode clamping must be

included to keep inputs within an acceptable range.

Figure 36 to Figure 39 show circuits for most common fan

TACH outputs. If the fan TACH output has a resistive pull-up

to VCC, it can be connected directly to the fan input, as shown

in Figure 36.

Figure 35. Inte

Single N-Channel MOSFET

Because the MOSFET can handle up to 3.5 A, it is simpl

matter of connecting another fan directly in parallel with

first. Care s

required to sou

8 mA maximum current specified on the data sheet.

Driving up to Three Fans from PWM3

PWM channels; for example, TACH1 is synchronized to

PWM1. TACH3 and TACH4 are both synchronized to PWM3,

so PWM3 can drive two fans. Alternatively, PWM3 can be pro-

grammed to synchronize TACH2, TACH3, and TACH4 to the

PWM3 output. This allows PWM3 to drive two or three fans.

In this case, the drive circuitry looks the same, as sho

Figure 37 and Figure 38. The SYNC bit in Register 0x62 ena

this function.

Synchronization is not required in high frequency mode when

used with 4-w

12V

VCC

PULL-UP

4.7kΩ

TYPICAL

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7475

Figure 36. Fan with TACH Pull-Up to V

ADT7475 Datasheet(PDF) 27 Page - Analog Devices

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