ADT7460ARQZ-RL7 ONSEMI [ON Semiconductor], ADT7460ARQZ-RL7 Datasheet - Page 24

• Manufacturer Part Number: ADT7460ARQZ-RL7
• Description: dBCOOL Remote Thermal Monitor and Fan Controller
• Manufacturer: ONSEMI [ON Semiconductor]

Driving Two Fans from PWM3
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 42 shows how to drive two fans in parallel using
low cost NPN transistors. Figure 43 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.
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.
Driving Up to Three Fans from PWM2
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 41,
Figure 42, and Figure 43. The SYNC bit in Register 0x62
enables this function.
ADT7460
Note that the ADT7460 has four TACH inputs available
Care should be taken in designing drive circuits with
TACH measurements for fans are synchronized to
Figure 42. Interfacing Two Fans in Parallel to the
PWM3 Output Using Low Cost NPN Transistors
Figure 41. Driving a 3−Wire Fan Using an NPN
PWM3
ADT7460
TACH/AIN
3.3V
PWM
1kΩ
2.2kΩ
3.3V
4.7kΩ
470Ω
Transistor
10kΩ
TACH3
Q1
MMBT3904
3.3V
10Ω
10Ω
10kΩ
TACH
12V
Q2
MMBT2222
12V
Q1
MMBT2222
12V
FAN
12V
1N4148
http://onsemi.com
TACH4
ADT7460
24
Driving 2−Wire Fans
the ADT7460. This circuit allows the speed of a 2−wire fan
to be measured, even though the fan has no dedicated TACH
signal. A series resistor, R
the fan commutation pulses into a voltage. This is
ac−coupled into the ADT7460 through the 0.01 mF
capacitor. On−chip signal conditioning allows accurate
monitoring of fan speed. The value of R
depends on the programmed input threshold and on the
current drawn by the fan. For fans drawing approximately
200 mA, a 2 W R
is programmed as 40 mV. For fans that draw more current,
such as larger desktop or server fans, R
reduced for the same programmed threshold. The smaller
the threshold programmed the better, since more voltage is
developed across the fan and the fan spins faster. Figure 45
shows a typical plot of the sensing waveform at a
TACH/AIN pin. The most important thing is that the voltage
spikes (either negative going or positive going) are more
than 40 mV in amplitude. This allows fan speed to be
reliably determined.
Table 26. SYNC: Enhance Acoustics Register 1
(Reg. 0x62)
<4>
Bit
ADT7460
Figure 44 shows how a 2−wire fan may be connected to
PWM3 Output Using a Single N−Channel MOSFET
Figure 43. Interfacing Two Fans in Parallel to the
TACH4
TACH3
PWM3
ADT7460
Mnemonic
SYNC
TACH/AIN
Figure 44. Driving a 2−Wire Fan
3.3V
3.3V
3.3V
PWM
10kΩ
TYPICAL
10kΩ
TYPICAL
10kΩ
TYPICAL
SENSE
TACH
3.3V
1 synchronizes TACH2, TACH3, and
TACH4 to PWM3.
10kΩ
TYPICAL
value is suitable when the threshold
+V
Q1
NDT3055L
SENSE
0.01μF
12V FAN
5.0V OR
5V OR
12V FAN
, in the fan circuit converts
Description
+V
1N4148
Q1
NDT3055L
R
2Ω
TYPICAL
TACH
SENSE
SENSE
SENSE
+V
1N4148
may be
chosen
5V OR
12V FAN