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REV. A

ADM1027

–27–

AUTOMATIC FAN SPEED CONTROL MODE

The ADM1027 has a local temperature sensor and two remote

temperature channels that may be connected to an on-chip

diode-connected transistor on a CPU. These three temperature

channels may be used as the basis for automatic fan speed control

to drive fans using pulsewidth modulation (PWM). In general,

the greater the number of fans in a system, the better the cooling,

but to the detriment of system acoustics. Automatic fan speed

control reduces acoustic noise by optimizing fan speed according

to 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 param-

eters, including TMIN and TRANGE, as discussed in detail later.

The TMIN and TRANGE values chosen for a given fan are critical,

since these define the thermal characteristics of the system. The

thermal validation of the system is one of the most important

steps of the design process, so these values should be carefully

selected.

The aim of this section is not only to provide the system designer

with an understanding of the automatic fan control loop, but also

to provide step-by-step guidance as to how to most effectively

evaluate and select the critical system parameters. To optimize

the system characteristics, the designer needs to give some fore-

thought to how the system will be configured, e.g., 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 actual system evaluation should

also be involved at the beginning of the process.

Automatic Fan Control Overview

Figure 27 gives a top-level overview of the automatic fan control

circuitry on the ADM1027. 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 ADM1027 allows the speed

of four fans to be monitored. The right side of the block diagram

shows controls that are fan-specific. The designer has control

over individual parameters such as minimum PWM duty cycle,

fan speed failure thresholds, and even ramp control of the PWM

outputs. This ultimately allows graceful fan speed changes that

are less perceptible to the system user.

TACHOMETER 1

MEASUREMENT

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT

TACHOMETER 2

MEASUREMENT

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT

TACHOMETER 3

AND 4

MEASUREMENT

PWM

GENERATOR

PWM

CONFIG

PWM

GENERATOR

PWM

CONFIG

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT

PWM

GENERATOR

PWM

CONFIG

PWM

MIN

PWM

MIN

PWM

MIN

MUX

THERMAL CALIBRATION

100%

TMIN

TRANGE

THERMAL CALIBRATION

100%

TMIN

TRANGE

THERMAL CALIBRATION

100%

TMIN

TRANGE

PWM1

PWM2

PWM3

REMOTE 1

TEMP

LOCAL

TEMP

REMOTE 2

TEMP

Figure 27. Automatic Fan Control Block Diagram

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