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ADM1031
display the correct fan speed, and also to program the correct
count value in RPM feedback mode.
into the ADM1031 through the 0.01 m F capacitor. On-chip
signal conditioning allows accurate monitoring of fan speed.
Fan Speed Measurement Equations
For a 4-pole fan (2 tach pulses/rev):
Fan RPM + (f 60) Count N
For a 6-pole fan (3 tach pulses/rev):
(eq. 14)
For typical notebook fans drawing approximately 170 mA,
a 2 W R SENSE value is suitable. For fans such as desktop or
server fans that draw more current, R SENSE can be reduced.
The smaller R SENSE is, the better, since more voltage is
developed across the fan, and the fan then spins faster.
Fan RPM + (f
60) (Count
N
1.5)
(eq. 15)
+V
For an 8-pole fan (4 tach pulses/rev):
Fan RPM + (f 60) (Count N
2)
(eq. 16)
3.3 V
5 V OR 12 V
FAN
If in doubt as to the number of poles the fans used have,
or the number of tach output pulses/rev, consult the fan
manufacturer ’s data sheet, or contact the fan vendor for
more information.
Fan Drive Using PWM Control
The external circuitry required to drive a fan using PWM
control is extremely simple. A single NMOS FET is the only
PWM_OUT
ADM1031
TACH/AIN
10 k W
TYPICAL
TACH
0.01 m F
Q1
NDT3055L
R SENSE
2 W TYPICAL
drive transistor required. The specifications of the MOSFET
depend on the maximum current required by the fan being
driven. Typical notebook fans draw a nominal 170 mA, and
so SOT devices can be used where board space is a
constraint. If driving several fans in parallel from a single
PWM output, or driving larger server fans, the MOSFET
needs to handle the higher current requirements. The only
other stipulation is that the MOSFET should have a gate
voltage drive, V GS < 3.3 V, for direct interfacing to the
PWM_OUT pin. The MOSFET should also have a low
Figure 35. Interfacing the ADM1031 to a 2-wire Fan
Figure 36 shows a typical plot of the sensing waveform at
the TACH/AIN pin. The most important thing is that the
negative-going spikes are more than 250 mV in amplitude.
This is the case for most fans when R SENSE = 2 W . The value
of R SENSE can be reduced as long as the voltage spikes at the
TACH/AIN pin are greater than 250 mV. This allows fan
speed to be reliably determined.
on-resistance to ensure that there is not significant voltage
drop across the FET. This would reduce the maximum
operating speed of the fan.
Tek PreVu
T
T
D: 250m
@: –258
Figure 34 shows how a 3-wire fan can be driven using
PWM control.
TACH/AIN
3.3 V
10 k W
TYPICAL
3.3 V
TACH
+V
5 V OR 12 V
FAN
1
ADM1031
PWM_OUT
10 k W
TYPICAL
Q1
NDT3055L
4
CH1 100mV
CH3 50.0mV
CH2 5.00mV
CH4 50.0mV
M 4.00ms
A CH1
–2.00mV
Figure 36. Fan Speed Sensing Waveform at
Figure 34. Interfacing the ADM1031 to a 3-wire Fan
The NDT3055L n-type MOSFET was chosen since it has
3.3 V gate drive, low on-resistance, and can handle 3.5 A of
current. Other MOSFETs can be substituted based on the
system’s fan drive requirements.
Figure 35 shows how a 2-wire fan can be connected to the
ADM1031. This circuit allows the speed of the 2-wire fan to
be measured even though the fan has no dedicated Tach
signal. A series R SENSE resistor in the fan circuit converts
the fan commutation pulses into a voltage. This is accoupled
TACH/AIN Pin
Fan Speed Measurement
The fan counter does not count the fan tach output pulses
directly, because the fan speed can be less than 1000 RPM
and it would take several seconds to accumulate a
reasonably large and accurate count. Instead, the period of
the fan revolution is measured by gating an on-chip
11.25 kHz oscillator into the input of an 8-bit counter. The
fan speed measuring circuit is initialized on the rising edge
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