ADT7476AARQZ-R datasheet(28/69 Pages) ONSEMI

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ADT7476A

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Figure 35 uses a 10 k

W pullup resistor for the TACH

signal. This assumes that the TACH signal is an

open-collector from the fan. In all cases, the TACH signal

from the fan must be kept below 5.5 V maximum to prevent

damaging the ADT7476A.

Figure 36 shows a fan drive circuit using an NPN

transistor such as a general-purpose MMBT2222. While

these devices are inexpensive, they tend to have much lower

current handling capabilities and higher on resistance than

MOSFETs. When choosing a transistor, care should be taken

to ensure that it meets the fan’s current requirements. Ensure

that the base resistor is chosen so that the transistor is

saturated when the fan is powered on.

Because the fan drive circuitry in 4-wire fans is not

switched on or off, as with previous PWM driven/powered

fans, the internal drive circuit is always on and uses the

PWM input as a signal instead of a power supply. This

enables the internal fan drive circuit to perform better than

3-wire fans, especially for high frequency applications.

Figure 36. Driving a 3-wire Fan Using

an NPN Transistor

ADT7476A

TACH

PWM

MMBT2222

12 V

FAN

3.3 V

12 V

10 k

4.7 k

470

10 k

TACH

Figure 37 shows a typical drive circuit for 4-wire fans.

Figure 37. Driving a 4-wire Fan

ADT7476A

TACH

PWM

12 V, 4-WIRE FAN

12 V

10 k

4.7 k

10 k

TACH

VCC

TACH

PWM

3.3 V

2 k

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.

Table 34. SYNC: ENHANCE ACOUSTICS REGISTER 1

(REG. 0x62)

Bit

Mnemonic

Description

[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

ADT7476A

PWM3

MMBT3904

3.3 V

1 k

TACH4

2.2 k

3.3 V

TACH3

10 k

12 V

MMBT2222

3.3 V

Figure 39. Interfacing Two Fans in Parallel to the

PWM3 Output Using a Single N-channel MOSFET

ADT7476A

TACH4

NDT3055L

3.3 V

10 k

TYP

TACH3

PWM3

3.3 V

10 k

TYP

10 k

TYP

5 V

12 V

FAN

TACH

5 V

12 V

FAN

TACH

3.3 V

ADT7476AARQZ-R Datasheet(PDF) 28 Page - ON Semiconductor

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