ADT7463
http://onsemi.com
29
The maximum input signal range is 0 V to 5 V, even where
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
resistive attenuation of the fan signal or diode clamping
must be included to keep inputs within an acceptable range.
Figures 47 to 50 show circuits for most common fan
TACH outputs.
If the fan TACH output has a resistive pullup to V
CC
, it
can be connected directly to the fan input, as shown in
Figure 47.
Figure 47. Fan with TACH Pullup to V
CC
12 V
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH
PULLUP
4.7 kW
TYP
ADT7463
If the fan output has a resistive pullup to 12 V (or other
voltage greater than 5 V) then the fan output can be clamped
with a Zener diode, as shown in Figure 48. The Zener diode
voltage should be chosen so that it is greater than V
IH
of the
TACH input but less than 5 V, allowing for the voltage
tolerance of the Zener. A value of between 3 V and 5 V is
suitable.
Figure 48. Fan with TACH Pullup to Voltage > 5.0 V,
e.g., 12 V, Clamped with Zener Diode
12 V
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH
PULLUP
4.7 kW
TYP
ADT7463
ZD1*
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ?V
CC
If the fan has a strong pullup (less than 1 kW) to 12 V or
a totempole output, then a series resistor can be added to
limit the Zener current, as shown in Figure 49. Alternatively,
a resistive attenuator may be used, as shown in Figure 50. R1
and R2 should be chosen such that:
(eq. 3)
2V t V
PULLUP
R2(R
PULLUP
) R1 ) R2) t 5 V
The fan inputs have an input resistance of nominally
160 kW to ground, so this 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 would be 100 kW
and 47 kW. This gives a high input voltage of 3.83 V.
Figure 49. Fan with Strong TACH. Pullup to > V
CC
or
TotemPole Output, Clamped with Zener and Resistor
5.0 V or12 V
V
CC
FAN SPEED
COUNTER
TACH
ADT7463
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 ?V
CC
PULLUP
TYP < 1 kW
OR TOTEMPOLE
ZD1*
ZENER
TACH
OUTPUT
R1
10 kW
FAN
Figure 50. Fan with Strong TACH. Pullup to > V
CC
or
TotemPole Output, Attenuated with R1/R2
12 V
V
CC
FAN SPEED
COUNTER
TACH
ADT7463
*SEE TEXT
< 1 kW
R1*
R2*
TACH
OUTPUT
Fan Speed Measurement
The fan counter does not count the fan TACH output
pulses directly because the fan speed may 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 revolution is measured by gating an onchip 90 kHz
oscillator into the input of a 16bit counter for N periods of
the fan TACH output (Figure 51), so the accumulated count
is actually proportional to the fan tachometer period and
inversely proportional to the fan speed.
Figure 51. Fan Speed Measurement
1
2
3
4
CLOCK
PWM
TACH
N, the number of pulses counted, is determined by the
settings of Register 0x7B (fan pulses per revolution
register). This register contains two bits for each fan,
allowing one, two (default), three, or four TACH pulses to
be counted.
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