adt7476a Analog Devices, Inc., adt7476a Datasheet - Page 30

• Manufacturer Part Number: adt7476a
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

ADT7476A
Figure 37 shows a typical drive circuit for 4-wire fans.
Driving Two Fans from PWM3
The ADT7476A 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.
ADT7476A
TACH
PWM
Figure 37. Driving a 4-Wire Fan
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
ADT7476A
ADT7476A
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
3.3V
Rev. 0 | Page 30 of 72
3.3V
Q1
MMBT3904
TACH3
10kΩ
10kΩ
+V
Q1
NDT3055L
5V OR
12V FAN
Because the MOSFET can handle up to 3.5 A, users can
connect 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 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 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.
(SYNC) Enhance Acoustics Register 1 (0x62)
[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
.