ADT7463ARQZ-R7 datasheet(26/52 Pages) ONSEMI

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REV. C

–26–

ADT7463

PWM3

2.2k�

1k�

3.3V

MMBT3904

3.3V

10�

TACH3

MMBT2222

10�

TACH4

12V

MMBT2222

Figure 31. Interfacing Two Fans in Parallel to the PWM3 Output Using Low Cost NPN Transistors

ADT7463

TACH4

10k�

TYPICAL

3.3V

TACH

TACH3

10k�

TYPICAL

3.3V

PWM3

10k�

TYPICAL

3.3V

NDT3055L

5V OR

12V FAN

TACH

5V OR 12V

FAN

1N4148

Figure 32. Interfacing Two Fans in Parallel to the PWM3 Output Using a Single N-Channel MOSFET

Driving Two Fans from PWM3

Note that the ADT7463 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 31 shows

how to drive two fans in parallel using low cost NPN transistors.

Figure 32 is the equivalent circuit using the NDT3055L MOSFET.

Note that since 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 data sheet.

Driving up to Three Fans from PWM2

TACH measurements for fans are synchronized to particular

PWM channels, e.g., TACH1 is synchronized to PWM1. TACH3

and TACH4 are both synchronized to PWM3 so PWM3 can

drive 2 fans. Alternatively, PWM2 can be programmed to

synchronize TACH2, TACH3, and TACH4 to the PWM2 out-

put. This allows PWM2 to drive two or three fans. In this case,

the drive circuitry looks the same as shown in Figures 31 and 32.

The SYNC bit in Register 0x62 enables this function.

<4> (SYNC) ENHANCE ACOUSTICS REG 1 (0X62)

SYNC = 1 Synchronizes TACH2, TACH3, and TACH4

to PWM2.

Rev. 4 | Page 26 of 52 | www.onsemi.com

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