REV. A
38
ADM1027
CHOOSING 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. Each ramp rates effects should be logged, if
possible, to determine the best setting for a given solution.
Enhanced Acoustics Register 1 (Reg. 0x62)
<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
Enhanced Acoustics Register 2 (Reg. 0x63)
<2:0> ACOU3 Select the ramp rate for PWM3.
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
<6:4> ACOU2 Select the ramp rate for PWM2.
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
Another way to view the ramp rates is as the time it takes for the
PWM output to ramp from 0% to 100% duty cycle for an instan-
taneous change in temperature. This can be tested by putting
the ADM1027 into manual mode and changing the PWM output
from 0% to 100% PWM duty cycle. The PWM output takes
35 sec to reach 100% with a ramp rate of 1 time slot selected.
TIME sec
140
0
0.76
120
100
80
60
40
20
0
120
100
80
60
40
20
0
R
TEMP
(C)
POWER DUTY CYCLE (%)
Figure 40. Enhanced Acoustics Mode with Ramp Rate = 48
Figure 40 shows remote temperature plotted against PWM duty
cycle for enhanced acoustics mode. The ramp rate is set to 48,
which corresponds to the fastest ramp rate. Assume that a new
temperature reading is available every 115 ms. With these set-
tings, it took approximately 0.76 seconds to go from 33% duty
cycle to 100% duty cycle (full speed). It can be seen that even
though the temperature increased very rapidly, the fan ramps up
to full speed gradually.
Figure 41 shows how changing the ramp rate from 48 to 8
affects the control loop. The overall response of the fan is
slower. Since the ramp rate is reduced, it takes longer for the
fan to achieve full running speed. In this case, it took approxi-
mately 4.4 sec for the fan to reach full speed.
TIME sec
120
0
4.4
140
120
100
80
60
40
0
100
80
60
40
20
0
20
Figure 41. Enhanced Acoustics Mode with Ramp Rate = 8
As can be seen from the preceding examples, the rate at which
the fan will react to temperature change is dependent on the
ramp rate selected in the enhance acoustics registers. The higher
the ramp rate, the faster the fan will reach the newly calculated
fan speed.
Figure 42 shows the behavior of the PWM output as temperature
varies. As the temperature rises, the fan speed ramps up. Small
drops in temperature will not affect the ramp-up function since
the newly calculated fan speed will still be higher than the previous
PWM value. The enhanced acoustics mode allows the PWM
output to be made less sensitive to temperature variations. This
will be dependent on the ramp rate selected and programmed
into the enhanced acoustics.
90
90
80
70
60
50
40
20
80
70
60
50
40
0
30
10
30
20
10
0
R
TEMP
PWM DUTY CYCLE
Figure 42. How Fan Reacts to Temperature
Variation in Enhanced Acoustics Mode
Rev. 3 | Page 38 of 56 | www.onsemi.com
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