ADM1030ARQ-REEL View Datasheet(PDF) - ON Semiconductor
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ADM1030ARQ-REEL Datasheet PDF : 29 Pages
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ADM1030
1. Run the fan at 53% PWM duty cycle in Software Mode. Clear
Bits 5 and 7 of Configuration Register 1 (Reg 0x00) to enter
PWM duty cycle mode. Write 0x08 to the Fan Speed Config
Register (Reg 0x22) to set the PWM output to 53% duty cycle.
2. Measure the fan RPM. This represents the fan RPM below
which the RPM mode will fail to operate. Do NOT program a
lower RPM than this value when using RPM Feedback mode.
3. Ensure that Speed Range, N, = 2 when using RPM Feed-
back mode.
Fans come in a variety of different options. One distinguishing
feature of fans is the number of poles that a fan has internally.
The most common fans available have four, six, or eight poles.
The number of poles the fan has generally affects the number of
pulses per revolution the fan outputs.
If the ADM1030 is used to drive fans other than 4-pole fans that
output 2 tach pulses/revolution, then the fan speed measurement
equation needs to be adjusted to calculate and display the cor-
rect fan speed, and also to program the correct count value in
RPM Feedback Mode.
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):
Fan RPM = (f ¥ 60)/(Count ¥ N ¥ 1.5)
For an 8-pole fan (4 tach pulses/rev):
Fan RPM = (f ¥ 60)/(Count ¥ N ¥ 2)
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 con-
trol is extremely simple. A single NMOS FET is the only drive
transistor required. The specifications of the MOSFET depend
on the maximum current required by the fan being driven. Typi-
cal 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 will need to handle the higher current
requirements. The only other stipulation is that the MOSFET
should have a gate voltage drive, VGS < 3.3 V, for direct inter-
facing to the PWM_OUT pin. The MOSFET should also have
a low on-resistance to ensure that there is not significant volt-
age drop across the FET. This would reduce the maximum
operating speed of the fan.
Figure 18 shows how a 3-wire fan may be driven using
PWM control.
TACH/AIN
ADM1030
PWM_OUT
3.3V
10k�
TYPICAL
TACH
3.3V
10k�
TYPICAL
+V
5V OR 12V
FAN
Q1
NDT3055L
Figure 18. Interfacing the ADM1030 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 may be substituted based on the system’s fan
drive requirements.
+V
PWM_OUT
3.3V
10k�
TYPICAL
TACH
ADM1030
5V OR 12V
FAN
Q1
NDT3055L
TACH/AIN
0.01�F
RSENSE
(2� TYPICAL)
Figure 19. Interfacing the ADM1030 to a 2-Wire Fan
Figure 19 shows how a 2-wire fan may be connected to the
ADM1030. This circuit allows the speed of the 2-wire fan to
be measured even though the fan has no dedicated Tach sig-
nal. A series RSENSE resistor in the fan circuit converts the fan
commutation pulses into a voltage. This is ac-coupled into
the ADM1030 through the 0.01 mF capacitor. On-chip signal
conditioning allows accurate monitoring of fan speed. For typical
notebook fans drawing approximately 170 mA, a 2 W RSENSE
value is suitable. For fans such as desktop or server fans, that
draw more current, 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 20 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 will be the case for most fans when
RSENSE = 2 W. The value of RSENSE 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.
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