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12V
PULL-UP
4.7kΩ
TACH
TYPICAL
OUTPUT
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
Figure 38. Fan with TACH Pull-Up to Voltage > 3.6 V
(for example, 12 V) Clamped with Zener Diode
If the fan has a strong pull-up (less than 1 k Ω ) to 12 V or a
totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 39.
12V
PULL-UP
4.7kΩ OR
TYPICAL
TACH
OUTPUT
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
Figure 39. Fan with Strong TACH Pull-Up to > V
Clamped with Zener and Resistor
Alternatively, a resistive attenuator can be used, as shown in
Figure 40. R1 and R2 should be chosen such that
2 V < V
× R2/(R
PULL-UP
PULL-UP
The fan inputs have an input resistance of nominally 160 k Ω to
ground, which should be taken into account when calculating
resistor values.
With a pull-up voltage of 12 V and pull-up resistor less than
1 kΩ, suitable values for R1 and R2 are 120 kΩ and 47 kΩ,
respectively. This gives a high input voltage of 3.35 V.
12V
PULL-UP
4.7kΩ
TYPICAL
Figure 37. Fan with TACH Pull-Up to V
V
CC
TACH
FAN SPEED
COUNTER
ZD1*
ADT7473
×
V
CC
V
CC
TACH
FAN SPEED
COUNTER
ZD1
ZENER*
ADT7473
×
V
CC
or Totem-Pole Output,
CC
+ R1 + R2) < 3.6 V
Rev. 0 | Page 27 of 76
V
CC
FAN SPEED
COUNTER
TACH
TACH
OUTPUT
ADT7473
CC
12V
<1kΩ
Figure 40. Fan with Strong TACH Pull-Up to > V
Fan Speed Measurement
The fan counter does not count the fan TACH output pulses
directly, because the fan speed could be less than 1,000 RPM
and it would take several seconds to accumulate a reasonably
large and accurate count. Instead, the period of the fan revolu-
tion 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 41), 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 the TACH pulses per revolution register (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
R1*
TACH
FAN SPEED
COUNTER
TACH
R2*
OUTPUT
ADT7473
*SEE TEXT
or Totem-Pole Output,
CC
Attenuated with R1/R2
4
Figure 41. Fan Speed Measurement
ADT7473
V
CC
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