ADM1030
http://onsemi.com
22
Figure 35. Interfacing the ADM1030 to a 2-wire Fan
5 V OR 12 V
FAN
TACH/AIN
TACH
PWM_OUT
10 kW
TYPICAL
3.3 V
+V
Q1
NDT3055L
0.01 mF
R
SENSE
2 W TYPICAL
ADM1030
Figure 35 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
signal. 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 R
SENSE
value is suitable. For
fans such as desktop or server fans, that draw more current,
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 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 will be 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.
CH1 100mV
CH3 50.0mV
CH2 5.00mV
CH4 50.0mV
M 4.00ms A CH1 2.00mV
CH1
1
4
T
T
Tek PreVu
D: 250mV
@: 258mV
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 may 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 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 will repeat
until monitoring is disabled. The fan speed measurement is
stored in the Fan Speed Reading register at address 0x08.
The fan speed count is given by:
(eq. 17)
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 Bits 7 and 6 of Fan
Characteristics Register 1 (20h) as shown in Table 15.
Figure 37 shows how the fan measurements relate to the
PWM_OUT pulse trains.
Table 15. OSCILLATOR FREQUENCIES
Bit 7
Bit 6
N
Oscillator Frequency (kHz)
0
0
1
11.25
0
1
2
5.625
1
0
4
2.812
1
1
8
1.406
Figure 37. Fan Speed Measurement
CLOCK
CONFIG 2
REG. BIT 2
FAN
INPUT
FAN
MEASUREMENT
PERIOD
START OF
MONITORING
CYCLE
In situations where different output drive circuits are used
for fan drive, it may be desirable to invert the PWM drive
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