ADM1027 datasheet(22/56 Pages) AD

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REV. A

–22–

ADM1027

Driving 2-Wire Fans

Figure 22 shows how a 2-wire fan may be connected to the

ADM1027. 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. This is ac-coupled into the

ADM1027 through the 0.01

F capacitor. On-chip signal con-

ditioning allows accurate monitoring of fan speed. For fans

drawing approximately 200 mA, a 2

W RSENSE value is suitable.

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

server fans, RSENSE may be reduced. The smaller RSENSE is the

better, since more voltage will be developed across the fan, and

the fan will spin faster. Figure 23 shows a typical plot of the

sensing waveform at a TACH/AIN pin. The most important

thing is that the negative going spikes are more than 250 mV in

amplitude. This allows fan speed to be reliably determined.

ADM1027

TACH/AIN

PWM

10k

TYPICAL

3.3V

NDT3055L

5V OR 12V

FAN

RSENSE

TYPICAL

0.01 F

Figure 22. Driving a 2-Wire Fan

Ch1

100mV

Ch2 5.00mV

M 4.00ms A Ch1

–2.00mV

Ch3 50.0mV

Ch4 50.0mV

: +250mV

@: –258mV

–1.00000ms

Tek PreVu

[]

Figure 23. Fan Speed Sensing Waveform at

TACH/AIN Pin

Laying Out for 2-Wire and 3-Wire Fans

Figure 24 shows how to lay out a common circuit arrangement

for 2-wire and 3-wire fans. Some components will not be popu-

lated depending on whether a 2-wire or 3-wire fan is being used.

3.3V OR 5V

TACH/AIN

PWM

12V OR 5V

MMBT2222

FOR 3-WIRE FANS:

POPULATE R1, R2, R3

R4 = 0

C1 = UNPOPULATED

FOR 2-WIRE FANS:

POPULATE R4, C1

R1, R2, R3 UNPOPULATED

Figure 24. Planning for 2-Wire or 3-Wire Fans on a PCB

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