ADT7476AARQZ 查看數據表（PDF） - ON Semiconductor
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ADT7476AARQZ Datasheet PDF : 67 Pages
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ADT7476A
If the fan output has a resistive pullup to 12 V, or other
voltage greater than 5.5 V, the fan output can be clamped
with a Zener diode, as shown in Figure 42. The Zener diode
voltage should be chosen so that it is greater than VIH of the
TACH input but less than 5.5 V, allowing for the voltage
tolerance of the Zener. A value between 5.0 V and 5.5 V is
suitable.
12V
VCC
PULLUP
4.7kΩ
TYPICAL
TACH
OUTPUT TACH
ZD1*
FAN SPEED
COUNTER
ADT7476A
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × VCC
Figure 42. Fan with Strong TACH Pullup to > 5.5 V,
(for Example, 12 V) Clamped with Zener Diode
If the fan has a strong pullup (less than 1 kW) to 12 V or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 43.
5V OR 12V
VCC
FAN
PULLUP TYP
<1kΩ OR
TOTEM POLE
R1
10kΩ
TACH
OUTPUT
TACH
ZD1
ZENER*
FAN SPEED
COUNTER
ADT7476A
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × VCC
Figure 43. Fan with Strong TACH Pullup to >VCC
or Totem−Pole Output, Clamped with
Zener Diode and Resistor
Alternatively, a resistive attenuator can be used, as shown
in Figure 44. R1 and R2 should be chosen such that
2 V t VPULLUP R2ńǒRPULLUP ) R1 ) R2Ǔ t 5.5 V
(eq. 4)
The fan inputs have an input resistance of nominally
160 kW to ground, which should be taken into account when
calculating resistor values.
With a pullup voltage of 12 V and pullup resistor less than
1 kW, suitable values for R1 and R2 are 100 kW and 40 kW,
respectively. This gives a high input voltage of 3.42 V.
12V
VCC
<1kΩ
R1*
TACH
OUTPUT
TACH
R2*
FAN SPEED
COUNTER
ADT7476A
*SEE TEXT
Figure 44. Fan with Strong TACH Pullup to >VCC or
Totem−Pole Output, Attenuated with R1/R2
The fan counter does not count the fan TACH output
pulses directly because the fan speed could be less than
1000 RPM, and it takes several seconds to accumulate a
reasonably large and accurate count. Instead, the period of
the fan revolution is measured by gating an on-chip 90 kHz
oscillator into the input of a 16-bit counter for N periods of
the fan TACH output (Figure 45), so the accumulated count
is actually proportional to the fan tachometer period and
inversely proportional to the fan speed.
N, the number of pulses counted, is determined by the
settings of TACH pulses per revolution register (0x7B). This
register contains two bits for each fan, allowing one, two
(default), three, or four TACH pulses to be counted.
CLOCK
PWM
TACH 1
2
3
4
Figure 45. Fan Speed Measurement
Fan TachometerReading Registers
The fan tachometer readings are 16-bit values consisting
of a 2−byte read from the ADT7476A.
Register 0x28, TACH1 Low Byte = 0x00 default
Register 0x29, TACH1 High Byte = 0x00 default
Register 0x2A, TACH2 Low Byte = 0x00 default
Register 0x2B, TACH2 High Byte = 0x00 default
Register 0x2C, TACH3 Low Byte = 0x00 default
Register 0x2D, TACH3 High Byte = 0x00 default
Register 0x2E, TACH4 Low Byte = 0x00 default
Register 0x2F, TACH4 High Byte = 0x00 default
Reading Fan Speed from the ADT7476A
The measurement of fan speeds involves a 2-register read
for each measurement. The low byte should be read first.
This causes the high byte to be frozen until both high and
low byte registers have been read, preventing erroneous
TACH readings. The fan tachometer reading registers report
back the number of 11.11 ms period clocks (90 kHz
oscillator) gated to the fan speed counter from the rising
edge of the first fan TACH pulse to the rising edge of the
third fan TACH pulse (assuming two pulses per revolution
are being counted).
Because the device is essentially measuring the fan TACH
period, the higher the count value, the slower the fan is
actually running. A 16-bit fan tachometer reading of
0xFFFF indicates that either the fan has stalled or is running
very slowly (<100 RPM).
High Limit: > Comparison Performed
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