ADT7476 Analog Devices, Inc., ADT7476 Datasheet - Page 33

• Manufacturer Part Number: ADT7476
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

LAYING OUT 3-WIRE FANS
Figure 40 shows how to lay out a common circuit arrangement
for 3-wire fans.
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
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
connected directly to the fan input, as shown in Figure 41.
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
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.
12V
Figure 40. Planning for 3-Wire Fans on a PCB
PULL-UP
TACH
Figure 41. Fan with TACH Pull-Up to V
4.7kΩ
TYP
R1
R2
R3
TACH
OUTPUT
12V OR 5V
Q1
MMBT2222
TACH
IH
1N4148
of the TACH input but
3.3V OR 5V
R4
PWM
FAN SPEED
ADT7476
COUNTER
CC
V
CC
CC
, it can be
CC
is
Rev. B | Page 33 of 72
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.
Alternatively, a resistive attenuator can be used, as shown in
Figure 44. R1 and R2 should be chosen such that
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.
FAN
5V OR 12V
Figure 43. Fan with Strong TACH Pull-Up to > V
Figure 44. Fan with Strong TACH Pull-Up to > V
Figure 42. Fan with TACH Pull-Up to Voltage > 3.6 V (for Example, 12 V),
12V
2 V < V
12V
PULL-UP TYP
TOTEM POLE
PULL-UP
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
<1kΩ OR
PULL-UP
TYPICAL
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
Clamped with Zener Diode and Resistor
<1kΩ
4.7kΩ
× R2/(R
Clamped with Zener Diode
Attenuated with R1/R2
TACH
OUTPUT
TACH
OUTPUT
TACH
OUTPUT
10kΩ
R1
R1*
PULL-UP
*SEE TEXT
ZD1*
+ R1 + R2) < 3.6 V
TACH
ZD1
ZENER*
R2*
TACH
TACH
CC
CC
or Totem-Pole Output,
or Totem-Pole Output,
FAN SPEED
ADT7476
COUNTER
FAN SPEED
ADT7476
FAN SPEED
ADT7476
COUNTER
COUNTER
V
CC
V
V
CC
CC
ADT7476
CC
CC