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ADT7490
Actual Changes in PWM Output
(Advanced Acoustics Settings)
While the automatic fan control algorithm describes the
general response of the PWM output, it is also necessary to
note that the enhanced acoustics registers (0x62, 0x63, and
0x3C) can be used to set/clamp the maximum rate of change
of PWM output for a given temperature zone. This means that
if T RANGE is programmed with an AFC slope that is quite
steep, a relatively small change in temperature could cause a
The graphs in Figure 57 assume the fan starts from 0%
PWM duty cycle. Clearly, the minimum PWM duty cycle,
PWM MIN , needs to be factored in to see how the loop
actually performs in the system. Figure 58 shows how
T RANGE is affected when the PWM MIN value is set to 20%.
It can be seen that the fan actually runs at about 45% fan
speed when the temperature exceeds T MIN .
100 2 5 C
large change in PWM output and possibly an audible change
in fan speed, which can be noticeable/annoying to end users.
Decreasing the speed of the PWM output changes by
programming the smoothing on the appropriate temperature
90
80
70
2.5 5 C
3.33 5 C
4 5 C
5 5 C
6.67 5 C
70
50
30
channels (Register 0x62 and Register 0x63) changes how
fast the fan speed increases/decreases in the event of a
temperature spike. Slowly the PWM duty cycle increases
until the PWM duty cycle reaches the appropriate duty cycle
as defined by the AFC curve.
Figure 57 shows PWM duty cycle vs. temperature for
each T RANGE setting. Figure 57B shows how each T RANGE
setting affects fan speed vs. temperature. As can be seen
from the graph, the effect on fan speed is nonlinear.
100 2 5 C
2.5 5 C
90
3.33 5 C
80 4 5 C
5 5 C
6.67 5 C
60 8 5 C
10 5 C
13.3 5 C
40 16 5 C
20 5 C
26.6 5 C
20 32 5 C
4 0 5 C
10
53.3 5 C
0 80 5 C
0 20 40 60 80 100 120
TEMPERATURE ABOVE T MIN
(A)
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
0
0
20
20
40 60 80 100
TEMPERATURE ABOVE T MIN
(A)
40 60 80 100
120
120
8 5 C
10 5 C
13.3 5 C
16 5 C
20 5 C
26.6 5 C
32 5 C
40 5 C
53.3 5 C
80 5 C
2 5 C
2.5 5 C
3.33 5 C
4 5 C
5 5 C
6.67 5 C
8 5 C
10 5 C
13.3 5 C
16 5 C
20 5 C
26.6 5 C
32 5 C
40 5 C
53.3 5 C
80 5 C
100
90
80
70
2 5 C
2.5 5 C
3.33 5 C
4 5 C
5 5 C
6.67 5 C
TEMPERATURE ABOVE T MIN
(B)
Figure 58. T RANGE and % Fan Speed Slopes with
PWM MIN = 20%
60
50
40
30
20
10
0
0
20 40 60 80 100 120
TEMPERATURE ABOVE T MIN
(B)
Figure 57. T RANGE vs. Actual Fan Speed
(Not PWM Drive) Profile
8 5 C
10 5 C
13.3 5 C
16 5 C
20 5 C
26.6 5 C
32 5 C
4 0 5 C
53.3 5 C
80 5 C
Step 7: T THERM for Temperature Channels
T THERM is the absolute maximum temperature allowed
on a temperature channel. For PECI temperature channels,
the equivalent parameter is T CONTROL . Above this
temperature, a component such as the CPU or VRM may be
operating beyond its safe operating limit. When the
temperature measured exceeds T THERM , all fans are driven
at 100% PWM duty cycle (full speed) to provide critical
system cooling.
The fans remain running at 100% until the temperature
drops below T THERM minus hysteresis, where hysteresis is
the number programmed into the hysteresis registers (0x6D
and 0x6E). The default hysteresis value is 4 ? C.
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