ADT7467ARQZ-R7 datasheet(28/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 28 of 77 | www.onsemi.com

ADT7467

PWM3

3.3V

12V

MMBT3904

MMBT2222

10Ω

2.2kΩ

1kΩ

TACH3

TACH4

Figure 37. Interfacing Two Fans in Parallel to the PWM3 Output Using Low Cost NPN Transistors

ADT7467

PWM3

TACH3

TACH4

3.3V

TACH

NDT3055L

1N4148

5V OR

12V FAN

5V OR

12V FAN

10kΩ

TYPICAL

10kΩ

TYPICAL

10kΩ

TYPICAL

Figure 38. Interfacing Two Fans in Parallel to the PWM3 Output Using a Single N-Channel MOSFET

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

5 mA maximum current specified on the data sheet.

Driving up to Three Fans from PWM3

TACH measurements for fans are synchronized to particular

PWM channels; for example, TACH1 is synchronized to

PWM1. TACH3 and TACH4 are both synchronized to PWM3;

therefore, PWM3 can drive two fans. Alternatively, PWM3 can

be programmed 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 is as shown in Figure 37

and Figure 38. The SYNC bit in Register 0x62 enables this

function.

Synchronization is not required in high frequency mode when

used with 4-wire fans.

<4> (SYNC) Enhanced Acoustics Register 1 (0x62)

SYNC = 1 synchronizes TACH2, TACH3, and TACH4 to PWM3.

Driving 2-Wire Fans

The ADT7467 can only support 2-wire fans when low frequency

PWM mode is selected in Configuration Register 5, Bit 2. If this

bit is not set to 1, the ADT7467 cannot measure the speed of

2-wire fans.

Figure 39 shows how a 2-wire fan can be connected to the

ADT7467. This circuit allows the speed of a 2-wire fan to be

measured, even though the fan has no dedicated TACH signal.

A series resistor, RSENSE, in the fan circuit converts the fan

commutation pulses into a voltage, which is ac-coupled into the

ADT7467 through the 0.01 μF capacitor. On-chip signal

conditioning allows accurate monitoring of fan speed. The

value of RSENSE depends on the programmed input threshold and

the current drawn by the fan. For fans drawing approximately

200 mA, a 2 Ω RSENSE value is suitable when the threshold is

programmed as 40 mV.

For fans that draw more current, such as larger desktop or

server fans, RSENSE can be reduced for the same programmed

threshold. The smaller the threshold programmed, the better,

because more voltage is developed across the fan and the fan

spins faster. Figure 40 shows a typical plot of the sensing

waveform at the TACHx pin.

ADT7467ARQZ-R7 Datasheet(PDF) 28 Page - ON Semiconductor

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