ADT7476ARQH ONSEMI [ON Semiconductor], ADT7476ARQH Datasheet - Page 27

• Manufacturer Part Number: ADT7476ARQH
• Description: dBCOOL Remote Thermal Controller and Voltage Monitor
• Manufacturer: ONSEMI [ON Semiconductor]

Driving Two Fans from PWM3
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.
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
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.
when used with 4−wire fans.
(SYNC) Enhance Acoustics Register 1 (0x62)
[4] SYNC = 1, synchronizes TACH2, TACH3, and TACH4
to PWM3.
ADT7476
The ADT7476 has four TACH inputs available for fan
Because the MOSFET can handle up to 3.5 A, users can
TACH measurements for fans are synchronized to
Synchronization is not required in high frequency mode
Figure 38. Interfacing Two Fans in Parallel to the
PWM3 Output Using Low Cost NPN Transistors
PWM3
3.3V
1kΩ
2.2kΩ
3.3V
3.3V
Q1
MMBT3904
TACH3
10kΩ
10kΩ
Q2
MMBT2222
MMBT2222
Q3
12V
http://onsemi.com
TACH4
3.3V
27
Laying Out 3−Wire Fans
arrangement for 3−wire fans.
TACH Inputs
TACH inputs) are high impedance inputs intended for fan
speed measurement.
slow rise and fall times typical of fan tachometer outputs.
The maximum input signal range is 0 V to 5.5 V, even though
V
fan outputs that exceed 0 V to 5.5 V, either resistive
attenuation of the fan signal or diode clamping must be
included to keep inputs within an acceptable range.
TACH outputs.
be connected directly to the fan input, as shown in Figure 41.
CC
Figure 40 shows how to lay out a common circuit
Pin 9, Pin 11, Pin 12, and Pin 14 (when configured as
Signal conditioning in the ADT7476 accommodates the
Figure 41 to Figure 44 show circuits for most common fan
If the fan TACH output has a resistive pullup to V
PWM3 Output Using a Single N−Channel MOSFET
ADT7476
Figure 39. Interfacing Two Fans in Parallel to the
Figure 40. Planning for 3−Wire Fans on a PCB
is 3.3 V. In the event that these inputs are supplied from
TACH4
TACH3
PWM3
12V
Figure 41. Fan with TACH Pullup to V
PULLUP
3.3V
3.3V
TACH
4.7k
10kΩ
TYPICAL
10kΩ
TYPICAL
10kΩ
TYPICAL
3.3V
TYP
W
TACH
3.3V
3.3V
R1
R2
R3
TACH
OUTPUT
12V OR 5V
+V
Q1
NDT3055L
5V OR
12V FAN
Q1
MMBT2222
TACH
1N4148
R4
3.3V OR 5V
1N4148
PWM
TACH
FAN SPEED
ADT7476
COUNTER
V
CC
CC
+V
CC
5V OR
12V FAN
, it can