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Increasing or decreasing PWM
although the fan control still follows the same PWM duty cycle
to temperature slope. The effective T
values can be calculated using Equation 2.
T
= T
+ (Max DC − Min DC) × T
MAX
MIN
where:
(Max DC − Min DC) × T
RANGE
See the Note on Fan Speed and PWM Duty Cycle.
Figure 56 shows PWM duty cycle vs. temperature for each
T
setting. The lower graph shows how each T
RANGE
affects fan speed vs. temperature. As indicated by the graph, the
effect on fan speed is nonlinear.
100
90
80
70
60
50
40
30
20
10
0
0
20
40
60
TEMPERATURE ABOVE T
100
90
80
70
60
50
40
30
20
10
0
0
20
40
60
TEMPERATURE ABOVE T
Figure 56. T
vs. Actual Fan Speed Profile
RANGE
changes the effective T
MIN
RANGE
for different PWM
RANGE
/170
RANGE
/170 is the effective T
value.
RANGE
setting
RANGE
2°C
2.5°C
3.33°C
4°C
5°C
6.67°C
8°C
10°C
13.3°C
16°C
20°C
26.6°C
32°C
40°C
53.3°C
80°C
80
100
120
MIN
2°C
2.5°C
3.33°C
4°C
5°C
6.67°C
8°C
10°C
13.3°C
16°C
20°C
26.6°C
32°C
40°C
53.3°C
80°C
80
100
120
MIN
,
The graphs in Figure 56 assume the fan starts from 0% PWM
duty cycle. Clearly, the minimum PWM duty cycle, PWM
needs to be factored in to see how the loop actually performs in
MIN
the system. Figure 57 shows how T
PWM
value is set to 20%. It can be seen that the fan actually
MIN
(2)
runs at about 45% fan speed when the temperature exceeds T
100
90
80
70
60
50
40
30
20
10
0
0
20
100
90
80
70
60
50
40
30
20
10
0
0
20
Figure 57. T
Example: Determining T
Channel
The following example shows how the different T
settings can be applied to three different thermal zones. In this
example, the following T
T
= 80°C for ambient temperature
RANGE
T
= 53.3°C for CPU temperature
RANGE
T
= 40°C for VRM temperature
RANGE
This example uses the mux configuration, described in Step 2:
Configuring the Mux, with the ADT7473 connected as shown
in Figure 58. Both CPU temperature and VRM temperature
drive the CPU fan connected to PWM1. Ambient temperature
drives the front chassis fan and rear chassis fan connected to
PWM2 and PWM3. The front chassis fan is configured to
run at PWM
MIN
run at PWM
MIN
PWM
= 10%.
MIN
Rev. 0 | Page 41 of 76
is affected when the
RANGE
40
60
80
100
TEMPERATURE ABOVE T
MIN
40
60
80
100
TEMPERATURE ABOVE T
MIN
and % Fan Speed Slopes with PWM
RANGE
for Each Temperature
RANGE
values apply:
RANGE
= 20%. The rear chassis fan is configured to
= 30%. The CPU fan is configured to run at
ADT7473
,
MIN
.
MIN
2°C
2.5°C
3.33°C
4°C
5°C
6.67°C
8°C
10°C
13.3°C
16°C
20°C
26.6°C
32°C
40°C
53.3°C
80°C
120
2°C
2.5°C
3.33°C
4°C
5°C
6.67°C
8°C
10°C
13.3°C
16°C
20°C
26.6°C
32°C
40°C
53.3°C
80°C
120
= 20%
MIN
and T
MIN
RANGE
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