Advertisement
Laying Out 3-Wire Fans
Figure 40 shows how to lay out a common circuit
arrangement for 3-wire fans.
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 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
is 3.3 V. In the event that these inputs are
CC
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
be connected directly to the fan input, as shown in Figure 41.
12 V
PULLUP
4.7 kW
TYP
Figure 41. Fan with TACH Pullup to V
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
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.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Arrow.com.
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
12 V or 5 V
R1
1N4148
3.3 V or 5 V
R2
R4
R3
Q1
MMBT2222
ADT7476A
TACH
OUTPUT
FAN SPEED
COUNTER
TACH
ADT7476A
12 V
PULLUP
Figure 42. Fan with Strong TACH Pullup to > 5.5 V,
PWM
(for Example, 12 V) Clamped with Zener Diode
If the fan has a strong pullup (less than 1 kW) to 12 V or
a totem-pole output, a series resistor can be added to limit the
Zener current, as shown in Figure 43.
5 V or12 V
FAN
OR TOTEM-POLE
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
Figure 43. Fan with Strong TACH. Pullup to > V
or Totem-Pole Output, Clamped with
, it can
CC
Alternatively, a resistive attenuator can be used, as shown
V
CC
in Figure 44. R1 and R2 should be chosen such that:
2 V t V
PULLUP
The fan inputs have an input resistance of nominally
160 kW 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 kW, suitable values for R1 and R2 are 100 kW and 40 kW,
respectively. This gives a high input voltage of 3.42 V.
CC
12 V
< 1 kW
of the
IH
Figure 44. Fan with Strong TACH. Pullup to > V
Totem-Pole Output, Attenuated with R1/R2
www.onsemi.com
29
4.7 kW
TACH
TYP
OUTPUT
TACH
ZD1*
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
R1
10 kW
TACH
ZD1*
PULLUP
TACH
ZENER
TYP < 1 kW
OUTPUT
Zener Diode and Resistor
) R1 ) R2 t 5.5 V
R2 R
PULLUP
TACH
OUTPUT
TACH
R1*
R2*
V
CC
ADT7476A
FAN SPEED
COUNTER
CC
V
CC
ADT7476A
FAN SPEED
COUNTER
CC
CC
(eq. 4)
V
CC
ADT7476A
FAN SPEED
COUNTER
*SEE TEXT
or
CC
Advertisement
