ADT7467ARQZ-R7 datasheet(58/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 58 of 77 | www.onsemi.com

Approaches to System Acoustic Enhancement

There are two different approaches to implementing system

acoustic enhancement: temperature-centric and fan-centric.

The ADT7467 uses the fan-centric approach.

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 characteris-

tics of the system to change. It also causes the system fans to run

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) use 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 to 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 up or down smoothly 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

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

thermal characterization of the control loop has been

optimized. Now 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.

Enabling Acoustic Enhancement for Each PWM Output

Enhanced Acoustics Register 1 (0x62)

<3> = 1 enables acoustic enhancement on PWM1 output.

Enhanced Acoustics Register 2 (0x63)

<7> = 1 enables acoustic enhancement on PWM2 output.

<3> = 1 enables acoustic enhancement on PWM3 output.

Effect of Ramp Rate on Enhanced Acoustics 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.

170

TIME SLOTS

PWM OUTPUT

(ONE PERIOD)

= 255 TIME SLOTS

PWM_OUT

33% DUTY

CYCLE

Figure 82. 33% PWM Duty Cycle, Represented in Time Slots

The ramp rates in the enhanced acoustics mode are selectable

from the values 1, 2, 3, 5, 8, 12, 24, and 48. The ramp rates are

discrete time slots. For example, if the ramp rate is 8, eight time

slots are added or subtracted to increase or decrease, respec-

tively, the PWM high duty cycle. Figure 83 shows how the

enhanced acoustics mode algorithm operates.

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

Figure 83. Enhanced Acoustics Algorithm

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 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.

A ramp rate of 1 corresponds to one time slot, which is 1/255 of

the PWM period. In enhanced acoustics mode, incrementing or

decrementing by 1 changes the PWM output by 1/255 × 100%.

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