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ADT7467
TACH Inputs
When configured as TACH inputs, Pin 4, Pin 6, Pin 7, and
Pin 9 are open-drain TACH inputs intended for fan speed
5 V or12 V
FAN
V CC
measurement.
Signal conditioning in the ADT7467 accommodates the
slow rise and fall times typical of fan tachometer outputs.
R1
10 k W
TACHx
ADT7467
FAN SPEED
The maximum input signal range is 0 V to 5 V, even when
V CC is less than 5 V. In the event that these inputs are
supplied from fan outputs that exceed 0 V to 5 V, either
PULL-UP
TYP < 1 k W
OR TOTEM-POLE
TACH
OUTPUT
ZD1*
ZENER
COUNTER
resistive attenuation of the fan signal or diode clamping
must be included to keep inputs within an acceptable range.
Figure 41 to Figure 44 show circuits for most common fan
TACH outputs. If the fan TACH output has a resistive
pull-up to V CC , it can be connected directly to the fan input,
as shown in Figure 41.
12 V V CC
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ? V CC
Figure 43. Fan with Strong TACH. Pull-up to > V CC or
Totem - Pole Output, Clamped with Zener 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 V PULLUP R2 (R PULLUP ) R1 ) R2) t 5 V (eq. 2)
PULL-UP
4.7 k W
TYP
TACH
OUTPUT
TACHx
ADT7467
FAN SPEED
COUNTER
The fan inputs have an input resistance of nominally
160 k W to ground, which should be taken into account when
calculating resistor values.
With a pull-up voltage of 12 V and a pull-up resistor of
less than 1 k W , suitable values for R1 and R2 are 100 k W and
47 k W , respectively. This gives a high input voltage of
3.83 V.
Figure 41. Fan with TACH Pull-up to V CC
If the fan output has a resistive pull-up to 12 V (or another
12 V
V CC
voltage that is greater than 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 the V IH of the TACH input but less than 5 V, allowing
for the voltage tolerance of the Zener. A value between 3 V
< 1 k W
TACH
OUTPUT
R1*
TACHx
R2*
ADT7467
FAN SPEED
COUNTER
and 5 V is suitable.
12 V
V CC
*SEE TEXT
Figure 44. Fan with Strong TACH. Pull-up to > V CC or
Totem-Pole Output, Attenuated with R1/R2
PULL-UP
4.7 k W
TYP
TACH
OUTPUT
ZD1*
TACHx
ADT7467
FAN SPEED
COUNTER
Fan Speed Measurement
The fan counter does not count the fan TACH output
pulses directly, because the fan speed could be less than
1000 RPM, and it would take several seconds to accumulate
a reasonably large and accurate count. Instead, the period of
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ? V CC
Figure 42. Fan with TACH Pull-up to Voltage > 5.0 V,
(for Example, 12 V), Clamped with Zener Diode
If the fan has a strong pull-up (less than 1 k W ) to 12 V or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 43.
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 (see Figure 45); therefore, the
accumulated count is actually proportional to the fan
tachometer period and inversely proportional to the fan speed.
The number of pulses counted, N, is determined by the
settings of Register 0x7B (TACH pulses per revolution
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