ADM1027 datasheet(21/56 Pages) AD

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REV. A

ADM1027

–21–

ADM1027

PWM3

2.2k

10k

3.3V

MMBT3904

3.3V

TACH3

MMBT2222

TACH4

12V

MMBT2222

Figure 20. Interfacing Two Fans in Parallel to the PWM3 Output Using Low Cost NPN Transistors

ADM1027

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

Figure 21. Interfacing Two Fans in Parallel to the PWM3 Output Using a Single N-Channel MOSFET

Driving 2 Fans From PWM3

Note that the ADM1027 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 20

shows how to drive two fans in parallel using low cost NPN

transistors. Figure 21 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 max cur-

rent specified on the data sheet.

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