ADT7476
Rev. B | Page 33 of 72
LAYING OUT 3-WIRE FANS
Figure 40 shows how to lay out a common circuit arrangement
for 3-wire fans.
05382-041
Q1
MMBT2222
R2
R1
R3
R4
PWM
1N4148
3.3V OR 5V
12V OR 5V
TACH
Figure 40. Planning for 3-Wire Fans on a PCB
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 ADT7476 accommodates the slow
rise and fall times typical of fan tachometer outputs. The maxi-
mum input signal range is 0 V to 3.6 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 3.6 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 pull-up to V
CC
, it can be
connected directly to the fan input, as shown in Figure 41.
05
38
2
-04
2
12V
V
CC
PULL-UP
4.7k
Ω
TYP
TACH
OUTPUT
FAN SPEED
COUNTER
TACH
ADT7476
Figure 41. Fan with TACH Pull-Up to V
CC
If the fan output has a resistive pull-up to 12 V, or other voltage
greater than 3.6 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 3.6 V, allowing for the voltage tolerance of the Zener. A
value of between 3 V and 3.6 V is suitable.
05
38
2-
0
43
12V
V
CC
PULL-UP
4.7k
Ω
TYPICAL
TACH
OUTPUT
FAN SPEED
COUNTER
TACH
ADT7476
ZD1*
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
CC
Figure 42. 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 kΩ) to 12 V or a
totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 43.
0
53
82
-0
44
5V OR 12V
V
CC
PULL-UP TYP
<1k
Ω
OR
TOTEM POLE
TACH
OUTPUT
FAN SPEED
COUNTER
TACH
ADT7476
ZD1
ZENER*
FAN
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
CC
R1
10k
Ω
Figure 43. Fan with Strong TACH Pull-Up to > V
CC
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 <
V
PULL-UP
×
R2
/(
R
PULL-UP
+
R1
+
R2
) < 3.6 V
The fan inputs have an input resistance of nominally 160 kΩ 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 100 kΩ and 40 kΩ,
respectively. This gives a high input voltage of 3.42 V.
05
38
2-
0
45
12V
V
CC
<1k
Ω
TACH
OUTPUT
FAN SPEED
COUNTER
TACH
ADT7476
R2*
*SEE TEXT
R1*
Figure 44. Fan with Strong TACH Pull-Up to > V
CC
or Totem-Pole Output,
Attenuated with R1/R2
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