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REV. A

ADM1027

–37–

Enabling Acoustic Enhancement for Each PWM Output

ENHANCED ACOUSTICS REGISTER 1 (Reg. 0x62)

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

ENHANCED ACOUSTICS REGISTER 2 (Reg. 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 will have a period, T, given by

the PWM drive frequency, f, since 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 incre-

ment in PWM duty cycle. A PWM signal of 33% duty cycle will

thus be high for 1/3

255 time slots and low for 2/3

255

time slots. Therefore, 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 38. 33% PWM Duty Cycle Represented in Time Slots

The ramp rates in enhanced acoustics mode are selectable

between 1, 2, 3, 5, 8, 12, 24, and 48. The ramp rates are actu-

ally discrete time slots. For example, if the ramp rate = 8, then

eight time slots will be 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 will be decreased by

eight time slots. Figure 39 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 39. 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,

then 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, then 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, it 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%.

It is important to note that when using the enhanced acoustics

mode, the fan spin-up should be disabled.

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