Analog Devices ADT7473 Manual page 54

ADT7473
Figure 74 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
settings, it takes approximately 0.76 sec to go from 33% duty
cycle to 100% duty cycle (full speed). Even though the
temperature increases very rapidly, the fan ramps up to full
speed gradually.
140
( ° C)
R
TEMP
120
100
80
60
PWM CYCLE (%)
40
20
0
0
TIME (s)
Figure 74. Enhanced Acoustics Mode with Ramp Rate = 48
Figure 75 shows how changing the ramp rate from 48 to 8
affects the control loop. The overall response of the fan is
slower. Because the ramp rate is reduced, it takes longer for the
fan to achieve full running speed. In this case, it takes
approximately 4.4 sec for the fan to reach full speed.
120
R (°C)
TEMP
100
80
60
40
20
0
0
TIME (s)
Figure 75. Enhanced Acoustics Mode with Ramp Rate = 8
120
100
80
60
40
20
0
0.76
140
120
100
80
PWM DUTY CYCLE (%)
60
40
20
0
4.4
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Figure 76 shows the PWM output response for a ramp rate of 2.
In this instance, the fan took about 17.6 sec to reach full
running speed.
140
R (°C)
TEMP
120
100
80
60
40
20
0
0
TIME (s)
Figure 76. Enhanced Acoustics Mode with Ramp Rate = 2
Figure 77 shows how the control loop reacts to temperature
with the slowest ramp rate. The ramp rate is set to 1, while all
other control parameters remain the same. With the slowest
ramp rate selected, it takes 35 sec for the fan to reach full speed.
120
R (°C)
TEMP
100
80
60
40
20
0
0
TIME (s)
Figure 77. Enhanced Acoustics Mode with Ramp Rate = 1
As Figure 74 to Figure 77 show, the rate at which the fan reacts
to temperature change is dependent on the ramp rate selected in
the enhanced acoustics registers. The higher the ramp rate, the
faster the fan reaches the newly calculated fan speed.
120
100
80
60
PWM DUTY CYCLE (%)
40
20
0
17.6
140
120
100
80
60
PWM DUTY CYCLE (%)
40
20
0
35