ADT7462ACPZ-R7 datasheet(32/81 Pages) ONSEMI

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ADT7462

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PWM Duty Cycle Registers

By reading the PWMx current duty cycle registers, the

user can keep track of the current duty cycle on each PWM

output, even when the fans are running in automatic fan

speed control mode or acoustic enhancement mode.

Figure 50. Control PWM Duty Cycle Manually with a

Resolution of 0.39%

VARY PWM DUTY

CYCLE WITH 8-BIT

RESOLUTION

Programming the Automatic Fan Speed Control Loop

Note that to better 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 51 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

°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 51 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. Automatic fan control, then, ultimately allows

graceful fan speed changes that are less perceptible to the

system user.

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