ADT7460 Analog Devices, Inc., ADT7460 Datasheet - Page 29

• Manufacturer Part Number: ADT7460
• Description: Dbcool? Remote Thermal Controller And Fan Controller
• Manufacturer: Analog Devices, Inc.

Laying Out 2-Wire and 3-Wire Fans
Figure 47 shows how to lay out a comm
for 2-wire and 3-wire fans. Some components are not populated,
depending on whether a 2-wire or 3-wire fan is used.
TACH Inputs
Pins 4, 6, 7, and 9 are open-drain TACH inputs for fan speed
measure
Signal conditioning in the ADT7460 accommodates the slow
rise and fall times typical of fan tachometer outputs. The maxi-
mum input signal range is 0 V to 5 V, even where V
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 ran
F
TACH outputs.
igure 48 to Figure 51 show circuits for most common fan
TACH/AIN
CH1
CH3 50.0mV
ment.
Figure 46. Fan Speed Sensing Waveform at TACH/AIN Pin
100mV
Figure 47. Planning for 2-Wire or 3-Wire Fans on a PCB
CH2 5.00mV
CH4 50.0mV
R1
R2
R3
ge.
C1
12V OR 5V
R4
M 4.00ms
T
Q1
MMBT2222
FOR 3-WIR
POPULATE
R4 = 0Ω
C1 = UNPO
FOR 2-WIRE FANS:
POPULAT
R1, R2, R3
–1.00000ms
1N4148
on circuit arrangement
3.3V OR 5V
R5
E R4, C1
E FANS:
UNPOPULAT
PULATED
R1, R2, R3
A CH1
@:
∆
: 250 mV
–258mV
PWM
CC
–2.00mV
ED
is less
Rev. C | Page 29 of 52
If
co
If the fan outp
greater than 5 V), the fan output can be
diode, as shown in Figure 49. The Zene
be greater than V of the TACH input but less than 5 V, allowing
for the voltage tolerance of the
and 5 V is suitable.
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 50. Alternatively, a resistive
attenuator may be used, as shown in Figure 51. R1 and R2
shou
The fan
ground.
re
With a pull-up voltage of 12 V an
1 kΩ, s
47 k
FAN
5V OR 12V
sistor values.
the fan TACH output has a resistive pull-up to V
nnected directly to the fan input, as shown in Figure 48.
Figure 50. Fan with Strong TACH Pull-Up to > V
Ω. This gives a high input v
Figure 49. Fan with TACH Pull-Up to Voltage . 5 V, for example, 12 V,
ld be c
2 V V
12V
uitable values for R1 and R2 w
inp
Th
12V
<
PULL-UP TYP
TOTEM POLE
hosen such t
is should be ta
uts have an inp
PULLUP
<1k OR
ut has a resistive pull-up to 12 V (or other voltage
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
PULL-UP
TYPICAL
PULL-UP
TYPICAL
Ω
Figure 48. Fan with TACH Pull-Up to V
IH
4.7kΩ
4.7kΩ
×R2/(R
Clamped with Zener and Resistor
Clamped with Zener Diode
TACH
OUTPUT
hat
10kΩ
TACH
OUTPUT
PULL
ken in
TACH
OUTPUT
R1
ut re
UP
TACH
Zener. A value of between 3 V
+ R1 + R2) < 5 V
sistance of nominally 16
oltage of 3.83 V.
to account when calcul
ZD1
ZENER*
d pull-up re
ZD1*
TACH
TACH
ould be 100 kΩ a
r diode voltage should
clamped with a Zener
CC
ADT7460
FAN SPEED
COUNTER
or Totem-Pole Outpu
V
ADT7460
FAN SPEED
sistor less than
ADT7460
FAN SPEED
CC
COUNTER
COUNTER
CC
V
V
CC
CC
CC
ADT7460
, it can be
CC
CC
nd
ating
0 kΩ to
t,