ADT7473 Analog Devices, Inc., ADT7473 Datasheet - Page 27

• Manufacturer Part Number: ADT7473
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

first. Care should be taken in designing drive circuits with
transistors and FETs to ensure the PWM pins are not require
to source current and that they sink less than the 8 mA
maximum current specified on the data sheet.
Driving up to Three Fans fro PWM3
TACH measurem
PWM channels, for example, TACH1 is
PWM1. TACH3 and TACH4 are both synchronized to PWM3
so PWM3 can drive two fans. Alternatively, PWM3 can be pro-
grammed to synchronize TACH2, T
PWM3 output. This allows PWM3 to drive two or three fans. In
this case, the drive circuitry looks the same, as shown in
Figure 35 and Figure 36. The SYNC bit in Register 0x62 enables
this function.
Synchronization is not required in high frequency mode when
used with 4-wire fans.
Bit [4] (SYNC) of Enhance Acoustics Register 1 (0x62)
SYNC = 1, synchronizes TACH2, TACH3, and TACH4 to
PWM3.
TACH Inputs
Pin 4, Pin 6, Pin 7, and Pin 9 (when configured as TACH
inputs) are open-drain TACH inputs intended for fan speed
measurement.
Signal conditioning in the ADT7473 accommodates the slow
rise and fall times typical of fan tachometer outputs. The
ADT7473
ADT7473
Figure 36. Interfacing Two Fans in Parallel to the PWM3 Output Using a
Figure 35. Interfacing Two Fans in Parallel to the PWM3 Output Using
PWM3
TACH4
TACH3
PWM3
3.3V
3.3V
3.3V
3.3V
1kΩ
2.2kΩ
10kΩ
TYPICAL
10kΩ
TYPICAL
10kΩ
TYPICAL
ents for fans are synchronized t
TACH
3.3V
3.3V
Single N-Channel MOSFET
Low Cost NPN Transistors
3.3V
Q1
MMBT3904
3.3V
10kΩ
10kΩ
TACH3
+V
Q1
NDT3055L
m
5V OR
12V FAN
ACH3, and TACH4 to the
Q2
MMBT2222
synchronized to
MMBT2222
1N4148
TACH
Q3
o particular
+V
12V
5V OR
12V FAN
TACH4
3.3V
Rev. A | Page 27 of 76
d
,
maximum input signal range is 0 V to 3.6 V. In th
these inputs are supplied from fan outputs that exce
3.6 V, either resistive attenuation of the fan signal
clamping must be included to keep inputs within
range.
Figure 37 to Figure 40 show circuits for most com
TACH outputs.
If the fan TACH output has a resistive pull-up to V
connected directly to the fan input, as shown in Fi
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 38. 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 Zene
value of between 3 V and 3.6 V is suitable.
If the fan has a strong pull-up (less than 1 k Ω ) to 12 V or a
totem-pole output, a series resistor can be added to limit t
Zener current, as shown in Figure 39.
Alternatively, a resistive attenuator can be used, as shown in
Figure 40. R1 and R2 should be chosen such that
Figure 39. Fan with Strong TACH Pull-Up to > V
12V
2 V < V
12V
12V
Figure 38. Fan w
PULL-UP
4.7kΩ OR
PULL-UP
TYPICAL
PULL-UP
TYPICAL
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8
PULL-UP
TYPICAL
Figure 37. Fan with TACH Pull-Up to V
4.7kΩ
4.7kΩ
Clamped with Zener and Resistor
× R2/(R
Clamped with Zener Diode
TACH
OUTPUT
ith TACH Pull-Up to Voltage > 3.6 V
TACH
OUTPUT
TACH
OUTPUT
PULL-UP
+ R1 + R2) < 3.6 V
ZD1
ZENER*
ZD1*
TACH
TACH
TACH
IH
of the TACH inpu
CC
or Totem-Pole Output,
FAN SPEED
ADT7473
COUNTER
FAN SPEED
ADT7473
FAN SPEED
ADT7473
COUNTER
COUNTER
CC
V
V
CC
V
e event that
an acceptable
or diode
CC
mon fan
CC
gure 37.
ADT7473
CC
×
×
ed 0 V to
V
V
, it can be
CC
CC
t but
he
r. A