ADT7462 datasheet(39/92 Pages) AD

39 / 92 page

ADT7462

Rev. 0 | Page 39 of 92

PROGRAMMING THE AUTOMATIC FAN SPEED

CONTROL LOOP

Note that to more efficiently understand the automatic fan

speed control loop, use of the ADT7462 evaluation board and

software is strongly recommended 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 system characteristics, the designer

needs to carefully plan system configuration, including the

number of fans, where they are located, and what temperatures 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 the process.

Automatic Fan Control Overview

The ADT7462 can automatically control the speed of fans based

upon the measured temperature. This is done independently

from CPU intervention once initial parameters are set up.

The ADT7462 has a local temperature sensor and up to three

remote temperature channels that can be connected to a CPU

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

other CPUs/GPUs). These four 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 validation of the system is one of the most

important steps in the design process, so these values should be

selected carefully.

Figure 50 gives a top-level overview of the automatic fan control

circuitry on the ADT7462. From a systems-level perspective, up

to four system temperatures can be monitored and used to

control four PWM outputs. The four PWM outputs can be used

to control up to eight fans. The ADT7462 allows the speed of

eight fans to be monitored. The Remote 1 and Remote 2

temperature channels have a thermal calibration block, allowing

the designer to individually configure the thermal characteristics of

those temperature channels. For example, the CPU fan can be

run when CPU temperature increases above 60°C and a chassis

fan can be run 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 50 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%

TMIN

TRANGE

REMOTE 1

TEMP

LOCAL

TEMP

REMOTE 2

TEMP

TACHOMETER 1

MEASUREMENT

PWM

CONFIG

PWM

MIN

PWM1

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

PWM

GENERATOR

TACHOMETER 2

MEASUREMENT

PWM

CONFIG

PWM

MIN

PWM2

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

PWM

GENERATOR

TACHOMETER 3

AND 4

MEASUREMENT

PWM

CONFIG

PWM

MIN

PWM3

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

PWM

GENERATOR

100%

TACH1

TACH2

TACH3

Figure 50. Automatic Fan Control Block Diagram

ADT7462 Datasheet(PDF) 39 Page - Analog Devices

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