ADT7490ARQZ-R7 datasheet(33/75 Pages) ONSEMI

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ADT7490

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Programming the Automatic Fan Speed Control Loop

To more efficiently understand the automatic fan speed

control loop, using the ADT7490 evaluation board and

software while reading this section is recommended.

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 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 system development process.

Manual Fan Control Overview

In unusual circumstances, it can be necessary to manually

control the speed of the fans. Because the ADT7490 has an

SMBus interface, a system can read back all necessary

voltage, fan speed, and temperature information, and use

this information to control the speed of the fans by writing

to the current PWM duty cycle register (0x30, 0x31, and

0x32) of the appropriate fan. Bits [7:5] of the PWMx

configuration registers (0x5C, 0x5D, and 0x5E) are used to

set fans up for manual control.

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 ADT7490 can automatically control the speed of fans

based on the measured temperature. This is done

independently of CPU intervention once the initial

parameters are set up.

The ADT7490 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 48 gives a top−level overview of the automatic fan

control circuitry on the ADT7490. 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

ADT7490 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,

users 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 48 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 ultimately allows graceful

fan speed changes that are less perceptible to the system user.

ADT7490ARQZ-R7 Datasheet(PDF) 33 Page - ON Semiconductor

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