ADT7475 datasheet(33/68 Pages) AD

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ADT7475

Rev. A | Page 33 of 68

PROGRAMMING THE AUTOMATIC FAN SPEED CONTROL LOOP

To more efficiently understand the automatic fan speed control

loop, it is strongly recommended to use the ADT7475 evalua-

tion board and software while reading this section.

This section provides the system designer with an understanding

of the automatic fan control loop, and provides step-by-step

guidance on effectively evaluating and selecting critical system

parameters. To optimize the system characteristics, the designer

needs to give some thought to system configuration, including

the number of fans, where they are located, and what tempera-

tures are being measured in the particular system.

The mechanical or thermal engineer who is tasked with the

system thermal characterization should also be involved at

the beginning of this process.

AUTOMATIC FAN CONTROL OVERVIEW

The ADT7475 can automatically control the speed of fans based

upon the measured temperature. This is done independently of

CPU intervention once initial parameters are set up.

The ADT7475 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 owing 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 vali-

dation of the system is one of the most important steps in the

design process, so select these values carefully.

Figure 43 gives a top-level overview of the automatic fan control

circuitry on the ADT7475. 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 ADT7475 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, one 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 43 shows controls that are fan-specific. 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.

MUX

THERMAL CALIBRATION

TMIN

TRANGE

THERMAL CALIBRATION

100%

TMIN

TRANGE

THERMAL CALIBRATION

100%

PWM

MIN

TMIN

TRANGE

REMOTE 1

TEMP

LOCAL

TEMP

REMOTE 2

TEMP

PWM1

TACH1

TACH2

TACH3

PWM

MIN

PWM2

PWM

MIN

PWM3

100%

PWM

GENERATOR

PWM

GENERATOR

TACHOMETER 3

AND 4

MEASUREMENT

TACHOMETER 1

MEASUREMENT

TACHOMETER 2

MEASUREMENT

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

PWM

GENERATOR

CONFIG

PWM

CONFIG

PWM

CONFIG

Figure 43. Automatic Fan Control Block Diagram

ADT7475 Datasheet(PDF) 33 Page - Analog Devices

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