adm1027 Analog Devices, Inc., adm1027 Datasheet - Page 22

• Manufacturer Part Number: adm1027
• Description: Dbcool? Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

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
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
For fans that draw more current, such as larger desktop or
server fans, R
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
SENSE
Figure 22. Driving a 2-Wire Fan
TACH/AIN
may be reduced. The smaller R
SENSE
PWM
, in the fan circuit converts the fan
3.3V
10k
TYPICAL
0.01 F
SENSE
+V
Q1
NDT3055L
R
2
TYPICAL
SENSE
value is suitable.
5V OR 12V
FAN
SENSE
is the
–22–
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.
Figure 24. Planning for 2-Wire or 3-Wire Fans on a PCB
Tek PreVu
1
4
Ch1
Ch3 50.0mV
Figure 23. Fan Speed Sensing Waveform at
TACH/AIN Pin
TACH/AIN
100mV
Ch2 5.00mV
Ch4 50.0mV
[
R1
R2
R3
C1
T
T
M 4.00ms A Ch1
T
12V OR 5V
R4
–1.00000ms
Q1
MMBT2222
FOR 3-WIRE FANS:
POPULATE R1, R2, R3
R4 = 0
C1 = UNPOPULATED
FOR 2-WIRE FANS:
POPULATE R4, C1
R1, R2, R3 UNPOPULATED
]
3.3V OR 5V
R5
–2.00mV
PWM
@: –258mV
: +250mV
REV. A