ADT7476 Analog Devices, Inc., ADT7476 Datasheet - Page 32

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

ADT7476
Figure 37 shows a typical drive circuit for 4-wire fans.
Driving Two Fans from PWM3
The ADT7476 has four TACH inputs available for fan speed
measurement but only three PWM drive outputs. If a fourth fan
is being used in the system, it should be driven from the PWM3
output in parallel with the third fan.
Figure 38 shows how to drive two fans in parallel using low cost
NPN transistors. Figure 39 shows the equivalent circuit using a
MOSFET.
ADT7476
TACH
PWM
Figure 37. Driving a 4-Wire Fan
3.3V
4.7kΩ
Figure 39. Interfacing Two Fans in Parallel to the PWM3 Output Using a Single N-Channel MOSFET
2kΩ
Figure 38. Interfacing Two Fans in Parallel to the PWM3 Output Using Low Cost NPN Transistors
10kΩ
3.3V
10kΩ
TACH
12V 12V
ADT7476
ADT7476
12V, 4-WIRE FAN
TACH4
TACH3
V
TACH
PWM
PWM3
PWM3
CC
3.3V
3.3V
3.3V
3.3V
10kΩ
TYPICAL
10kΩ
TYPICAL
10kΩ
TYPICAL
1kΩ
2.2kΩ
TACH
3.3V
3.3V
Rev. B | Page 32 of 72
3.3V
3.3V
TACH3
Q1
MMBT3904
+V
10Ω
10Ω
Q1
NDT3055L
5V OR
12V FAN
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 outputs are not
required to source current and that they sink less than the 5 mA
maximum current specified (see Table 1).
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 38
and Figure 39. The SYNC bit in Register 0x62 enables this
function.
Synchronization is not required in high frequency mode when
used with 4-wire fans.
Enhanced Acoustics Register 1 (0x62)
Bit 4 SYNC = 1, synchronizes TACH2, TACH3, and TACH4
to PWM3
.
Q2
MMBT2222
MMBT2222
1N4148
TACH
Q3
12V
+V
TACH4
5V OR
12V FAN
3.3V