ADT7475 Analog Devices, Inc., ADT7475 Datasheet - Page 27

• Manufacturer Part Number: ADT7475
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

Because 4-wire fans are powered continuously, the fan speed is
not switched on or off as with previous PWM driven/powered
fans. This enables it to perform better than 3-wire fans, espe-
cially for high frequency applications. Figure 33 shows a typical
drive circuit for 4-wire fans.
Driving Two Fans from PWM3
The ADT7475 has four TACH inputs available for fan speed
measurement but only three PWM drive outputs. If a fourth fan
is used in the system, it should be driven from the PWM3
output in parallel with the third fan. Figure 34 shows how to
drive two fans in parallel using low cost NPN transistors. Figure 35
shows the equivalent circuit using a MOSFET.
Figure 34. Interfacing Two Fans in Parallel to the PWM3 Output Using Low
Figure 35. Interfacing Two Fans in Parallel to the PWM3 Output Using a
ADT7475
ADT7475
TACH4
TACH3
PWM3
PWM3
ADT7475
3.3V
3.3V
3.3V
3.3V
10kΩ
TYPICAL
10kΩ
TYPICAL
10kΩ
TYPICAL
1kΩ
TACH
2.2kΩ
PWM
Figure 33. Driving a 4-Wire Fan
TACH
Single N-Channel MOSFET
Cost NPN Transistors
3.3V
+V
TACH3
4.7kΩ
Q1
MMBT3904
2kΩ
Q1
NDT3055L
10Ω
10Ω
10kΩ
3.3V
10kΩ
5V OR
12V FAN
TACH
12V 12V
Q2
MMBT2222
MMBT2222
1N4148
12V, 4-WIRE FAN
V
TACH
PWM
TACH
CC
Q3
+V
12V
5V OR
12V FAN
TACH4
Rev. B | Page 27 of 68
Because the MOSFET can handle up to 3.5 A, it is simply a
matter of connecting another fan directly in parallel with the
first. Care should be taken in designing drive circuits with
transistors and FETs to ensure that the PWM pins are not
required to source current and that they sink less than the
8 mA maximum current specified on the data sheet.
Driving up to Three Fans from PWM3
TACH measurements for fans are synchronized to particular
PWM channels; for example, TACH1 is synchronized to
PWM1. TACH3 and TACH4 are both synchronized to PWM3,
so PWM3 can drive two fans. Alternatively, PWM3 can be pro-
grammed to synchronize TACH2, TACH3, and TACH4 to the
PWM3 output. This allows PWM3 to drive two or three fans.
In this case, the drive circuitry looks the same, as shown in
Figure 37 and Figure 38. 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) Enhanced 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 ADT7475 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. In the event 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 36 to Figure 39 show circuits for most common fan
TACH outputs. If the fan TACH output has a resistive pull-up
to V
in Figure 36.
CC
, it can be connected directly to the fan input, as shown
12V
PULL-UP
TYPICAL
Figure 36. Fan with TACH Pull-Up to V
4.7kΩ
TACH
OUTPUT
TACH
FAN SPEED
ADT7475
COUNTER
B
CC
V
CC
ADT7475