ADT7476AARQZ-R datasheet(33/69 Pages) ONSEMI

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ADT7476A

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In high frequency mode, the PWM drive frequency is

always 22.5 kHz. When high frequency mode is enabled, the

dc bits are automatically asserted internally and do not need

to be changed.

Fan Speed Control

The ADT7476A controls fan speed using automatic and

manual modes:

• In automatic fan speed control mode, fan speed is

automatically varied with temperature and without CPU

intervention once initial parameters are set up. The

advantage is that if the system hangs, the user is

guaranteed that the system is protected from

overheating.

• In manual fan speed control mode, the ADT7476A

allows the duty cycle of any PWM output to be

adjusted manually. This can be useful if the user wants

to change fan speed in software or adjust PWM duty

cycle output for test purposes. Bits [7:5] of

control the behavior of each PWM output.

Table 42. PWM CONFIGURATION REGISTERS

(REG. 0x5C TO 0x5E)

Bit

Mnemonic

Description

[7:5]

BHVR

111 = Manual Mode

Once under manual control, each PWM output can be

manually

updated

writing

Programming the PWM Current Duty Cycle Registers

The PWM current duty cycle registers are 8-bit registers

that allow the PWM duty cycle for each output to be set

anywhere from 0% to 100% in steps of 0.39%. The value to

be programmed into the PWMMIN register is given by:

Value (decimal) = PWMMIN/0.39

Example 1:

For a PWM duty cycle of 50%,

Value (decimal) = 50/0.39 = 128 (decimal)

Value = 128 (decimal) or 0x80 (hex)

Example 2:

For a PWM duty cycle of 33%,

Value (decimal) = 33/0.39 = 85 (decimal)

Value = 85 (decimal) or 0x54 (hex)

Table 43. PWM CURRENT DUTY CYCLE REGISTERS

Description

Default

0x30

PWM1 Current Duty Cycle

0xFF (100%)

0x31

PWM2 Current Duty Cycle

0xFF (100%)

0x32

PWM3 Current Duty Cycle

0xFF (100%)

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.

Programming TRANGE

TRANGE defines the distance between TMIN and 100%

PWM. For the ADT7467, ADT7468 and ADT7473,

TRANGE is effectively a slope. For the ADT7475

andADT7476A, TRANGE is no longer a slope, but defines

the temperature region where the PWM output linearly

ramps from PWMMIN to 100% PWM.

Figure 46. TRANGE

TMIN

PWM = 100%

PWM = 0%

PWMMIN

PWMMAX

TRANGE

Programming the Automatic Fan Speed Control Loop

To understand the automatic fan speed control loop more

efficiently, it is recommended to use the ADT7476A

evaluation 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 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 ADT7476A 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 PWM current duty cycle register (0x30, 0x31, and

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

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

fans up for manual control.

ADT7476AARQZ-R Datasheet(PDF) 33 Page - ON Semiconductor

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