ADT7490ARQZ-R7 datasheet(29/75 Pages) ONSEMI

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ADT7490

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Figure 38. Driving a 4−Wire Fan

ADT7490

TACH

PWM

12V, 4−WIRE FAN

3.3V

12V 12V

4.7k

VCC

TACH

PWM

10kW

Driving Two Fans from PWM3

The ADT7490 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 39 shows how to drive two fans in parallel using

low cost NPN transistors. Figure 40 shows the equivalent

circuit using a MOSFET.

Because the MOSFET can handle up to 3.5 A, it is simply

a matter of connecting another fan directly in parallel with

the first. Care should be taken in designing drive circuits

with transistors and FETs to ensure the PWM outputs are not

required to source current, and that they sink less than the

5 mA maximum current specified in 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 39 and

Figure 40. The SYNC bit in Register 0x62 enables this

function.

Synchronization is not required in high frequency mode

when used with 4−wire fans.

SYNC, Enhanced Acoustics Register 1 (Register 0x62)

Bit 4 (SYNC) = 1, synchronizes TACH2, TACH3, and

TACH4 to PWM3.

Figure 39. Interfacing Two Fans in Parallel to the

PWM3 Output Using Low Cost NPN Transistors

ADT7490

PWM3

3.3V

12V

MMBT3904

MMBT2222

2.2k

TACH3

TACH4

3.3V

10kW

MMBT2222

ADT7490

PWM3

TACH3

TACH4

3.3V

TACH

NDT3055L

1N4148

5V OR

12V FAN

5V OR

12V FAN

10kΩ

TYPICAL

10kΩ

TYPICAL

10kΩ

TYPICAL

3.3V

Figure 40. Interfacing Two Fans in Parallel to the

PWM3 Output Using a Single N−Channel MOSFET

Laying Out 3−Wire Fans

Figure 41 shows how to lay out a common circuit

arrangement for 3−wire fans.

Figure 41. Planning for 3−Wire Fans on a PCB

MMBT2222

PWM

1N4148

3.3V OR 5V

12V OR 5V

TACH

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 ADT7490 accommodates the

slow rise and fall times typical of fan tachometer outputs. The

maximum input signal range is 0 V to 3.6 V, even though VCC

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

outputs that exceed 0 V to 3.6 V, either resistive attenuation

of the fan signal or diode clamping must be included to keep

inputs within an acceptable range.

Figure 42 to Figure 45 show circuits for the 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 42.

Figure 42. Fan with TACH Pullup to VCC

12V

VCC

PULLUP

4.7k

TYP

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7490

ADT7490ARQZ-R7 Datasheet(PDF) 29 Page - ON Semiconductor

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