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ADT7476A
http://onsemi.com
26
Figure 35 uses a 10 kW pullup resistor for the TACH
signal. This assumes that the TACH signal is an
open-collector from the fan. In all cases, the TACH signal
from the fan must be kept below 5.5 V maximum to prevent
damaging the ADT7476A.
Figure 36 shows a fan drive circuit using an NPN
transistor such as a general-purpose MMBT2222. While
these devices are inexpensive, they tend to have much lower
current handling capabilities and higher on resistance than
MOSFETs. When choosing a transistor, care should be taken
to ensure that it meets the fans current requirements. Ensure
that the base resistor is chosen so that the transistor is
saturated when the fan is powered on.
Because the fan drive circuitry in 4-wire fans is not
switched on or off, as with previous PWM driven/powered
fans, the internal drive circuit is always on and uses the
PWM input as a signal instead of a power supply. This
enables the internal fan drive circuit to perform better than
3-wire fans, especially for high frequency applications.
Figure 36. Driving a 3-wire Fan Using
an NPN Transistor
ADT7476A
TACH
PWM
Q1
MMBT2222
12 V
FAN
3.3 V
12 V
12 V
10 kW
4.7 kW
470 W
10 kW
TACH
Figure 37 shows a typical drive circuit for 4-wire fans.
Figure 37. Driving a 4-wire Fan
ADT7476A
TACH
PWM
12 V, 4-WIRE FAN
12 V
12 V
10 kW
4.7 kW
10 kW
TACH
V
CC
TACH
PWM
3.3 V
2 kW
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.
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 programmed 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.
Table 34. SYNC: ENHANCE ACOUSTICS REGISTER 1
(REG. 0x62)
Bit
Mnemonic
Description
[4]
SYNC
1, Synchronizes TACH2, TACH3, and
TACH4 to PWM3.
Figure 38. Interfacing Two Fans in Parallel to the
PWM3 Output Using Low Cost NPN Transistors
ADT7476A
PWM3
Q1
MMBT3904
3.3 V
1 kW
TACH4
2.2 kW
3.3 V
TACH3
10 kW
10 kW
12 V
Q2
MMBT2222
Q3
MMBT2222
3.3 V
3.3 V
Figure 39. Interfacing Two Fans in Parallel to the
PWM3 Output Using a Single N-channel MOSFET
ADT7476A
TACH4
Q1
NDT3055L
3.3 V
10 kW
TYP
TACH3
PWM3
3.3 V
3.3 V
10 kW
TYP
10 kW
TYP
+V
+V
5 V
or
12 V
FAN
TACH
5 V
or
12 V
FAN
TACH
3.3 V
3.3 V
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