ADT7476AARQZ-R datasheet(34/69 Pages) ONSEMI

34 / 69 page

ADT7476A

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THERM Operation in Manual Mode

In manual mode, if the temperature increases above the

programmed

THERM

temperature

limit,

the

fans

automatically speed up to maximum PWM or 100% PWM,

whichever way the appropriate fan channel is configured.

Automatic Fan Control Overview

The ADT7476A can automatically control the speed of

fans based on the measured temperature. This is done

independently of CPU intervention once initial parameters

are set up.

The ADT7476A has a local temperature sensor and two

remote temperature channels that can be connected to a CPU

on-chip thermal diode (available on Intel Pentium class and

other CPUs). These three temperature channels can be used

as the basis for automatic fan speed control to drive fans

using pulse-width modulation (PWM).

Automatic fan speed control reduces acoustic noise by

optimizing fan speed according to accurately measured

temperature. Reducing fan speed can also decrease system

current consumption. The automatic fan speed control mode

is very flexible due to the number of programmable

parameters, including TMIN and TRANGE. The TMIN and

TRANGE values for a temperature channel and, therefore, for

a given fan, are critical, because they define the thermal

characteristics of the system. The thermal validation of the

system is one of the most important steps in the design

process, so these values should be selected carefully.

Figure 47 gives a top-level overview of the automatic fan

control circuitry on the ADT7476A. From a systems-level

perspective, up to three system temperatures can be

monitored and used to control three PWM outputs. The three

PWM outputs can be used to control up to four fans. The

ADT7476A allows the speed of four fans to be monitored.

Each temperature channel has a thermal calibration block,

allowing the designer to individually configure the thermal

characteristics of each temperature channel. For example,

designers can decide to run the CPU fan when CPU

temperature increases above 60

°C and a chassis fan when

the local temperature increases above 45

°C.

At this stage, the designer has not assigned these thermal

calibration settings to a particular fan drive (PWM) channel.

The right side of Figure 47 shows fan-specific controls. The

designer has individual control over parameters such as

minimum PWM duty cycle, fan speed failure thresholds,

and even ramp control of the PWM outputs. Automatic fan

control, then, ultimately allows graceful fan speed changes

that are less perceptible to the system user.

Figure 47. Automatic Fan Control Block Diagram

THERMAL CALIBRATION

REMOTE1

TEMP

100%

TMIN

TRANGE

THERMAL CALIBRATION

100%

TMIN

TRANGE

THERMAL CALIBRATION

100%

TMIN

TRANGE

LOCAL

TEMP

REMOTE2

TEMP

PWM

MIN

PWM

MIN

PWM

MIN

MUX

PWM

CONFIG

PWM

GENERATOR

PWM1

PWM2

PWM3

TACH1

TACH2

TACH3

TACHOMETER 1

MEASUREMENT

TACHOMETER 2

MEASUREMENT

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

TACHOMETER 3

AND 4

MEASUREMENT

PWM

CONFIG

PWM

GENERATOR

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

PWM

CONFIG

PWM

GENERATOR

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

ADT7476AARQZ-R Datasheet(PDF) 34 Page - ON Semiconductor

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