ADT7462ACPZ-R7 datasheet(46/81 Pages) ONSEMI

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ADT7462

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Approaches to System Acoustic Enhancement

There are two different approaches to implementing

system acoustic enhancement: temperature−centric and

fan−centric.

The temperature−centric approach involves smoothing

transient temperatures as they are measured by a

temperature source (for example, Remote 1 temperature).

The temperature values used to calculate the PWM duty

cycle values are smoothed, reducing fan speed variation.

However, this approach causes an inherent delay in updating

fan speed and causes the thermal characteristics of the

system to change. It also causes the system fans to stay on

longer than necessary, because the fan’s reaction is merely

delayed. The user has no control over noise from different

fans driven by the same temperature source. Consider, for

example, a system in which control of a CPU cooler fan (on

PWM1) and a chassis fan (on PWM2) uses Remote 1

temperature. Because the Remote 1 temperature is

smoothed, both fans are updated at exactly the same rate. If

the chassis fan is much louder than the CPU fan, there is no

way to improve its acoustics without changing the thermal

solution of the CPU cooling fan.

The

fan−centric

approach

system

acoustic

enhancement controls the PWM duty cycle, driving the fan

at a fixed rate (for example, 6%). Each time the PWM duty

cycle is updated, it is incremented by a fixed 6%. As a result,

the fan ramps smoothly to its newly calculated speed. If the

temperature starts to drop, the PWM duty cycle immediately

decreases by 6% at every update. Therefore, the fan ramps

smoothly up or down without inherent system delay.

Consider, for example, controlling the same CPU cooler

fan (on PWM1) and chassis fan (on PWM2) using Remote

1 temperature. The TMIN and TRANGE settings have already

been defined in automatic fan speed control mode; that is,

thermal characterization of the control loop has been

optimized. The chassis fan is noisier than the CPU cooling

fan. Using the fan−centric approach, PWM2 can be placed

into acoustic enhancement mode independently of PWM1.

The acoustics of the chassis fan can, therefore, be adjusted

without affecting the acoustic behavior of the CPU cooling

fan, even though both fans are controlled by Remote 1

temperature. The fan−centric approach is how acoustic

enhancement works on the ADT7462.

Enabling Acoustic Enhancement for Each PWM

Output

Enhanced Acoustics Register 1 (0x1A)

Bit 0 (En1) = 1 enables acoustic enhancement on PWM1

output.

Bit 1 (En2) = 1 enables acoustic enhancement on PWM2

output.

Enhanced Acoustics Register 2 (0x1B)

Bit 0 (En3) = 1 enables acoustic enhancement on PWM3

output.

Bit 1 (En4) = 1 enables acoustic enhancement on PWM4

output.

Effect of Ramp Rate on Enhanced Acoustic Mode

The PWM signal driving the fan has a period, t, given by

the PWM drive frequency, f, because t = 1/f. For a given

PWM period, t, the PWM period is subdivided into 255

equal time slots. One time slot corresponds to the smallest

possible increment in the PWM duty cycle. A PWM signal

of 33% duty cycle is, therefore, high for 1/3

× 255 time slots

and low for 2/3

× 255 time slots. Therefore, a 33% PWM

duty cycle corresponds to a signal that is high for 85 time

slots and low for 170 time slots.

Figure 75. 33% PWM Duty Cycle Represented in

Time Slots

170

TIME SLOTS

PWM OUTPUT

(ONE PERIOD)

= 255 TIME SLOTS

PWM_OUT

33% DUTY

CYCLE

The ramp rates in the enhanced acoustics mode are

selectable from 1 to 8. The ramp rates are discrete time slots.

For example, if the ramp rate is 8, then eight time slots are

added to the PWM high duty cycle each time the PWM duty

cycle needs to be increased. If the PWM duty cycle value

needs to be decreased, it is decreased by eight time slots.

Figure 76 shows how the enhanced acoustics mode

algorithm operates.

Figure 76. Enhanced Acoustics Mode Algorithm

READ

TEMPERATURE

CALCULATE

NEW PWM

DUTY CYCLE

IS NEW PWM

VALUE >

VALUE?

INCREMENT

PWM VALUE

BY RAMP RATE

YES

DECREMENT

PWM VALUE

BY RAMP RATE

The enhanced acoustics mode algorithm calculates a new

PWM duty cycle based on the temperature measured. If the

new PWM duty cycle value is greater than the previous

PWM value, the previous PWM duty cycle value is

incremented by either 1, 2, 3, 5, 8, 12, 24, or 48 time slots,

depending on the settings of the enhanced acoustics

registers. If the new PWM duty cycle value is less than the

previous PWM value, the previous PWM duty cycle is

decremented by 1, 2, 3, 5, 8, 12, 24, or 48 time slots. Each

time the PWM duty cycle is incremented or decremented, its

value is stored as the previous PWM duty cycle for the next

comparison.

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