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, R
SENSE
, 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 R
SENSE
value is suitable.
For fans that draw more current, such as larger desktop or
server fans, R
SENSE
may be reduced. The smaller R
SENSE
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
+V
Q1
NDT3055L
5V OR 12V
FAN
R
SENSE
2
TYPICAL
0.01F
Figure 22. Driving a 2-Wire Fan
1
Ch1
4
100mV
Ch2 5.00mV M 4.00ms A Ch1
2.00mV
Ch3 50.0mV
Ch4 50.0mV
T
: +250mV
@: 258mV
1.00000ms
T
Tek PreVu
[
]
T
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
Q1
MMBT2222
R4
R5
R3
C1
R2
R1
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
Rev. 3 | Page 22 of 56 | www.onsemi.com
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