ADT7476A
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29
Laying Out 3-Wire Fans
Figure 40 shows how to lay out a common circuit
arrangement for 3-wire fans.
Figure 40. Planning for 3-wire Fans on a PCB
Q1
MMBT2222
TACH
12 V or 5 V
1N4148
PWM
R1
R2
R3
R4
3.3 V or 5 V
TACH Inputs
Pin 9, Pin 11, Pin 12, and Pin 14 (when configured as
TACH inputs) are high impedance inputs intended for fan
speed measurement.
Signal conditioning in the ADT7476A accommodates the
slow rise and fall times typical of fan tachometer outputs.
The maximum input signal range is 0 V to 5.5 V, even
though V
CC
is 3.3 V. In the event that these inputs are
supplied from fan outputs that exceed 0 V to 5.5 V, either
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 pullup to V
CC
, it can
be connected directly to the fan input, as shown in Figure 41.
Figure 41. Fan with TACH Pullup to V
CC
12 V
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH
PULLUP
4.7 k
W
TYP
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 V
IH
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.
Figure 42. Fan with Strong TACH Pullup to > 5.5 V,
(for Example, 12 V) Clamped with Zener Diode
12 V
V
CC
FAN SPEED
COUNTER
TACH
OUTPUT
TACH
PULLUP
4.7 k
W
TYP
ADT7476A
ZD1*
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
×
V
CC
If the fan has a strong pullup (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.
Figure 43. Fan with Strong TACH. Pullup to > V
CC
or Totem
-
Pole Output, Clamped with
Zener Diode and Resistor
5 V or12 V
V
CC
FAN SPEED
COUNTER
TACH
ADT7476A
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
×
V
CC
PULLUP
TYP < 1 k
W
OR TOTEM-POLE
ZD1*
ZENER
TACH
OUTPUT
FAN
R1
10 k
W
Alternatively, a resistive attenuator can be used, as shown
in Figure 44. R1 and R2 should be chosen such that:
(eq. 4)
2 V
t
V
PULLUP
R2
ń
ǒ
R
PULLUP
)
R1
)
R2
Ǔ
t
5.5 V
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 pullup voltage of 12 V and pullup resistor less than
1 k
W
, suitable values for R1 and R2 are 100 k
W
and 40 k
W
,
respectively. This gives a high input voltage of 3.42 V.
Figure 44. Fan with Strong TACH. Pullup to > V
CC
or
Totem-Pole Output, Attenuated with R1/R2
12 V
V
CC
FAN SPEED
COUNTER
TACH
ADT7476A
*SEE TEXT
< 1 k
W
R1*
R2*
TACH
OUTPUT
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