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ADT7473
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
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 T MIN and T RANGE settings have already
been defined in automatic fan speed control mode, that is,
thermal characterization of the control loop has been
optimized. Here, 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 ADT7473/
ADT7473 ? 1.
Enabling Acoustic Enhancement for Each PWM
Output
Enhanced acoustics Register 1 (0x62)
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.
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, 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%.
READ
TEMPERATURE
CALCULATE
NEW PWM
DUTY CYCLE
Bit 3 = 1, enables acoustic enhancement on PWM1 output
Enhanced acoustics Register 2 (0x63)
Bit 7 = 1, enables acoustic enhancement on PWM2 output
Bit 3 = 1, enables acoustic enhancement on PWM3 output
IS
NEW PWM
VALUE >
PREVIOUS
VALUE?
N O
DECREMENT
PREVIOUS
PWM VALUE
BY RAMP RATE
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.
PWM_OUT
33% DUTY
CYCLE
YES
INCREMENT
PREVIOUS
PWM VALUE
BY RAMP RATE
Figure 76. Enhanced Acoustics Algorithm
Step 12: Ramp Rate for Acoustic Enhancement
The optimal ramp rate for acoustic enhancement can be
found through system characterization after the thermal
optimization has been finished. The effect of each ramp rate
should be logged, if possible, to determine the best setting
85
170
for a given solution.
TIME SLOTS TIME SLOTS
PWM OUTPUT (ONE PERIOD)
= 255 TIME SLOTS
Figure 75. 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, then eight time slots are added to the PWM high duty
cycle each time the PWM duty cycle needs to be increased.
Enhanced Acoustics Register 1 (0x62)
Bits <2:0> ACOU, select the ramp rate for PWM1.
000 = 1 time slot = 35 sec
001 = 2 time slots = 17.6 sec
010 = 3 time slots = 11.8 sec
011 = 5 time slots = 7 sec
100 = 8 time slots = 4.4 sec
101 = 12 time slots = 3 sec
110 = 24 time slots = 1.6 sec
111 = 48 time slots = 0.8 sec
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