ADM1031ARQZ View Datasheet(PDF) - ON Semiconductor
Part Name
Description
Manufacturer
ADM1031ARQZ Datasheet PDF : 30 Pages
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ADM1031
Figure 34 shows how a 3−wire fan can be driven using
PWM control.
3.3 V
+V
TACH/AIN
ADM1031
PWM_OUT
10kW
TYPICAL
TACH
3.3 V
10kΩ
TYPICAL
5.0 V OR 12 V
FAN
Q1
NDT3055L
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 RSENSE resistor in the fan circuit converts
the fan commutation pulses into a voltage. This is accoupled
into the ADM1031 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 can be reduced.
The smaller RSENSE is, the better, since more voltage is
developed across the fan, and the fan then spins faster.
+V
PWM_OUT
3.3 V
10kW
TYPICAL
TACH
ADM1031
5.0 V OR 12 V
FAN
Q1
NDT3055L
TACH/AIN
0.01μF
RSENSE
(2W TYPICAL)
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 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.
Figure 36. Fan Speed Sensing Waveform at
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
of a PWM high output if fan speed measurement is enabled
(Bit 2 and Bit 3 of Configuration Register 2 = 1). It then starts
counting on the rising edge of the second tach pulse and
counts for two fan tach periods, until the rising edge of the
fourth tach pulse, or until the counter overranges if the fan
tach period is too long. The measurement cycle repeats until
monitoring is disabled. The fan speed measurement is stored
in the fan speed reading register at address 0×08, 0×09. The
fan speed count is given by:
Count = (f × 60)/R × N
where:
f = 11.25 kHz
R = fan speed in RPM.
N = speed range (either 1, 2, 4, or 8)
The frequency of the oscillator can be adjusted to suit the
expected running speed of the fan by varying N, the speed
range. The oscillator frequency is set by Bit 7 and Bit 6 of
Fan Characteristics Register 1 (0×20) and Fan
Characteristics Register 2 (0×21) as shown in Table 11.
Figure 37 shows how the fan measurements relate to the
PWM_OUT pulse trains.
Table 11. Oscillator Frequencies
Bit 7 Bit 6 N
0
0
1
0
1
2
1
0
4
1
1
8
Oscillator Frequency (kHz)
11.25
5.625
2.812
1.406
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