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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 fan's 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.
TACH
ADT7476A
PWM
Figure 36. Driving a 3-wire Fan Using
Figure 37 shows a typical drive circuit for 4-wire fans.
TACH
ADT7476A
PWM
Figure 37. Driving a 4-wire Fan
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
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12 V
12 V
10 kW
10 kW
12 V
FAN
TACH
4.7 kW
3.3 V
470 W
Q1
MMBT2222
an NPN Transistor
12 V
12 V
12 V, 4-WIRE FAN
10 kW
V
10 kW
CC
TACH
TACH
PWM
4.7 kW
3.3 V
2 kW
ADT7476A
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
[4]
SYNC
3.3 V
ADT7476A
1 kW
PWM3
2.2 kW
Figure 38. Interfacing Two Fans in Parallel to the
PWM3 Output Using Low Cost NPN Transistors
3.3 V
10 kW
TYP
TACH4
3.3 V
ADT7476A
10 kW
TYP
TACH3
3.3 V
10 kW
TYP
PWM3
Figure 39. Interfacing Two Fans in Parallel to the
PWM3 Output Using a Single N-channel MOSFET
www.onsemi.com
28
Description
1, Synchronizes TACH2, TACH3, and
TACH4 to PWM3.
12 V
3.3 V
TACH3
3.3 V
Q1
MMBT3904
Q2
MMBT2222
10 kW
Q3
10 kW
MMBT2222
+V
3.3 V
5 V
TACH
or
12 V
TACH
3.3 V
FAN
Q1
NDT3055L
TACH4
3.3 V
+V
5 V
or
12 V
FAN
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