ADT7476ARQZ-R7 datasheet(27/67 Pages) ONSEMI

27 / 67 page

ADT7476

http://onsemi.com

Driving Two Fans from PWM3

The ADT7476 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.

Figure 38. Interfacing Two Fans in Parallel to the

PWM3 Output Using Low Cost NPN Transistors

ADT7476

PWM3

3.3V

12V

MMBT3904

MMBT2222

10kΩ

2.2kΩ

1kΩ

TACH3

TACH4

3.3V

Figure 39. Interfacing Two Fans in Parallel to the

PWM3 Output Using a Single N−Channel MOSFET

ADT7476

PWM3

TACH3

TACH4

3.3V

TACH

NDT3055L

1N4148

5V OR

12V FAN

5V OR

12V FAN

10kΩ

TYPICAL

10kΩ

TYPICAL

10kΩ

TYPICAL

3.3V

Laying Out 3−Wire Fans

Figure 40 shows how to lay out a common circuit

arrangement for 3−wire fans.

Figure 40. 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 ADT7476 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.

Figure 41. Fan with TACH Pullup to VCC

12V

VCC

PULLUP

4.7k

TYP

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7476

ADT7476ARQZ-R7 Datasheet(PDF) 27 Page - ON Semiconductor

ADT7476ARQZ-R7 Datasheet(HTML) 27 Page - ON Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67

Datasheet Download

Link URL