adm1027 Analog Devices, Inc., adm1027 Datasheet - Page 21

• Manufacturer Part Number: adm1027
• Description: Dbcool? Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

Driving 2 Fans From PWM3
Note that the ADM1027 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 20
shows how to drive two fans in parallel using low cost NPN
transistors. Figure 21 is the equivalent circuit using the
REV. A
Figure 21. Interfacing Two Fans in Parallel to the PWM3 Output Using a Single N-Channel MOSFET
Figure 20. Interfacing Two Fans in Parallel to the PWM3 Output Using Low Cost NPN Transistors
ADM1027
ADM1027
TACH4
TACH3
PWM3
PWM3
3.3V
3.3V
3.3V
10k
TYPICAL
10k
TYPICAL
10k
TYPICAL
3.3V
10k
2.2k
TACH
3.3V
–21–
Q1
MMBT3904
TACH3
+V
10
Q1
NDT3055L
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 max cur-
rent specified on the data sheet.
10
5V OR 12V
FAN
Q2
MMBT2222
TACH
12V
+V
Q3
MMBT2222
TACH4
5V OR 12V
FAN
ADM1027