ADT7476AARQZ 查看數據表（PDF） - ON Semiconductor

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ADT7476AARQZ

dBCOOL® Remote Thermal Controller and Voltage Monitor

ON Semiconductor 

ADT7476A

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.

(SYNC) Enhance Acoustics Register 1 (0x62)

[4] SYNC = 1, synchronizes TACH2, TACH3, and TACH4

to PWM3.

12V

ADT7476A 3.3V

PWM3

1kΩ

2.2kΩ

3.3V

TACH3

3.3V

Q1

MMBT3904

10kΩ

10kΩ

Q2

MMBT2222

Q3

MMBT2222

TACH4

3.3V

Figure 38. Interfacing Two Fans in Parallel to the

PWM3 Output Using Low Cost NPN Transistors

3.3V

TACH4

ADT7476A

TACH3

10kΩ

TYPICAL

3.3V

3.3V

10kΩ

TYPICAL

TACH

3.3V

3.3V

10kΩ

TYPICAL

PWM3

+V

5V OR 1N4148

12V FAN

TACH

Q1

NDT3055L

+V

5V OR

12V FAN

Figure 39. Interfacing Two Fans in Parallel to the

PWM3 Output Using a Single N−Channel MOSFET

Laying Out 3-Wire Fans

Figure 40 shows how to lay out a common circuit

arrangement for 3-wire fans.

12V OR 5V

R1

1N4148

3.3V OR 5V

R2

R4

TACH

R3

PWM

Q1

MMBT2222

Figure 40. Planning for 3−Wire Fans on a PCB

TACH Inputs

Pin 9, Pin 11, Pin 12, and Pin 14 (when configured as

TACH inputs) are high impedance inputs intended for fan

speed measurement.

Signal conditioning in the ADT7476A accommodates the

slow rise and fall times typical of fan tachometer outputs.

The maximum input signal range is 0 V to 5.5 V, even though

VCC is 3.3 V. In the event that these inputs are supplied from

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.

Figure 41 to Figure 44 show circuits for most common fan

TACH outputs.

If the fan TACH output has a resistive pullup to VCC, it can

be connected directly to the fan input, as shown in Figure 41.

VCC

12V

PULLUP

4.7kW

TYP

TACH

OUTPUT

TACH

FAN SPEED

COUNTER

ADT7476A

Figure 41. Fan with TACH Pullup to VCC

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