ADT7476AARQZ-R datasheet(27/69 Pages) ONSEMI

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ADT7476A

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below the thermal limit. Because the temperature for that

channel is measured only once for every monitoring cycle,

after THERM asserts, it is guaranteed to remain low for at

least one monitoring cycle.

The THERM pin can be configured to assert low, if the

Remote 1, local, or Remote 2 THERM temperature limits

are exceeded by 0.25

°C. The THERM temperature limit

registers

are

and

respectively.

Setting

Bits [5:7]

Configuration Register 5 (0x7C) enables the THERM

output feature for the Remote 1, local, and Remote 2

temperature channels, respectively. Figure 34 shows how

the THERM pin asserts low as an output in the event of a

critical overtemperature.

Figure 34. Asserting THERM as an Output, Based on

Tripping THERM Limits

THERM LIMIT

MONITORING

CYCLE

TEMP

THERM

0.25

THERM LIMIT

An alternative method of disabling THERM is to program

the THERM temperature limit to –63

°C or less in Offset 64

mode, or −128

°C or less in twos complement mode; that is,

for THERM temperature limit values less than –63

°C or

–128

°C, respectively, THERM is disabled.

Enabling and Disabling THERM on individual Channels

THERM can be enabled/disabled for individual or

combinations of temperature channels using Bits [7:5] of

Configuration Register 5 (0x7C).

THERM Hysteresis

Setting Bit 0 of Configuration Register 7 (0x11) disables

THERM hysteresis.

If THERM hysteresis is enabled and THERM is disabled

(Bit 2 of Configuration Register 4, 0x7D), the THERM pin

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 ADT7476A 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 ADT7476A 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

ADT7476A

TACH

PWM

NDT3055L

12 V

FAN

3.3 V

12 V

10 k

4.7 k

10 k

TACH

ADT7476AARQZ-R Datasheet(PDF) 27 Page - ON Semiconductor

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