ADT7476ARQZ-R7 datasheet(26/67 Pages) ONSEMI

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ADT7476

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does not assert low when a THERM event occurs. If

THERM hysteresis is disabled and THERM is disabled

(Bit 2 of Configuration Register 4, 0x7D) and assuming the

appropriate pin is configured as THERM), the THERM pin

asserts low when a THERM event occurs.

If THERM and THERM hysteresis are both enabled, the

THERM output asserts as expected.

THERM Operation in Manual Mode

In manual mode, THERM events do not cause fans to go

to full speed, unless Bit 3 of Configuration Register 6 (0x10)

is set to 1.

Additionally, Bit 3 of Configuration Register 4 (0x7D)

can be used to select the PWM speed on a THERM event

(100% or maximum PWM).

Bit 2 in Configuration Register 4 (0x7D) can be set to

disable THERM events from affecting the fans.

Fan Drive Using PWM Control

The ADT7476 uses pulse−width modulation (PWM) to

control fan speed. This relies on varying the duty cycle (or

on/off ratio) of a square wave applied to the fan to vary the

fan speed. The external circuitry required to drive a fan using

PWM control is extremely simple. For 4−wire fans, the

PWM drive might need only a pullup resistor. In many cases,

the 4−wire fan PWM input has a built−in, pullup resistor.

The ADT7476 PWM frequency can be set to a selection

of low frequencies or a single high PWM frequency. The

low frequency options are used for 3−wire fans, while the

high frequency option is usually used with 4−wire fans.

For 3−wire fans, a single N−channel MOSFET is the only

drive device required. The specifications of the MOSFET

depend on the maximum current required by the fan being

driven and the input capacitance of the FET. Because a

10 k

W (or greater) resistor must be used as a PWM pullup,

an FET with large input capacitance can cause the PWM

output to become distorted and adversely affect the fan

control range. This is a requirement only when using high

frequency PWM mode.

Typical notebook fans draw a nominal 170 mA, so SOT

devices can be used where board space is a concern. In

desktops, fans typically draw 250 mA to 300 mA each. If

you drive several fans in parallel from a single PWM output

or drive larger server fans, the MOSFET must handle the

higher current requirements. The only other stipulation is

that the MOSFET should have a gate voltage drive,

VGS < 3.3 V, for direct interfacing to the PWM output pin.

The MOSFET should also have a low on resistance to ensure

that there is not a significant voltage drop across the FET,

which would reduce the voltage applied across the fan and,

therefore, the maximum operating speed of the fan. Figure

35 shows how to drive a 3−wire fan using PWM control.

Figure 35. Driving a 3−Wire Fan Using an N−Channel

MOSFET

ADT7476

TACH

PWM

12V

FAN

NDT3055L

TACH

3.3V

12V

10kΩ

4.7kΩ

10kΩ

1N4148

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 ADT7476.

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

ADT7476

TACH

PWM

12V

FAN

MMBT2222

3.3V

12V

470Ω

4.7kΩ

10kΩ

1N4148

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

Figure 37. Driving a 4−Wire Fan

ADT7476

TACH

PWM

12V, 4−WIRE FAN

3.3V

12V 12V

2kΩ

4.7kΩ

10kΩ

VCC

TACH

PWM

ADT7476ARQZ-R7 Datasheet(PDF) 26 Page - ON Semiconductor

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