EVAL-ADT7463EB ON Semiconductor, EVAL-ADT7463EB Datasheet - Page 26

• Manufacturer Part Number: EVAL-ADT7463EB
• Manufacturer: ON Semiconductor

Specifications of EVAL-ADT7463EB

• Lead Free Status / RoHS Status: Supplier Unconfirmed

ADT7463
Driving Two Fans from PWM3
Note that the ADT7463 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 31 shows
how to drive two fans in parallel using low cost NPN transistors.
Figure 32 is the equivalent circuit using the NDT3055L MOSFET.
Note that since 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.
Figure 32. Interfacing Two Fans in Parallel to the PWM3 Output Using a Single N-Channel MOSFET
Figure 31. Interfacing Two Fans in Parallel to the PWM3 Output Using Low Cost NPN Transistors
ADT7463
ADT7463
TACH4
TACH3
PWM3
PWM3
3.3V
3.3V
3.3V
3.3V
10k
TYPICAL
10k
TYPICAL
10k
TYPICAL
1k
2.2k
TACH
12V FAN
5V OR
3.3V
Q1
MMBT3904
–26–
TACH3
10
+V
10
Q1
NDT3055L
Driving up to Three Fans from PWM2
TACH measurements for fans are synchronized to particular
PWM channels, e.g., TACH1 is synchronized to PWM1. TACH3
and TACH4 are both synchronized to PWM3 so PWM3 can
drive 2 fans. Alternatively, PWM2 can be programmed to
synchronize TACH2, TACH3, and TACH4 to the PWM2 out-
put. This allows PWM2 to drive two or three fans. In this case,
the drive circuitry looks the same as shown in Figures 31 and 32.
The SYNC bit in Register 0x62 enables this function.
<4> (SYNC) ENHANCE ACOUSTICS REG 1 (0X62)
SYNC = 1 Synchronizes TACH2, TACH3, and TACH4
to PWM2.
Q2
MMBT2222
1N4148
TACH
+V
12V
Q3
MMBT2222
TACH4
5V OR 12V
FAN
REV. C