Analog Devices dBCool ADT7473 Manual page 43

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
value.
See the Note on Fan Speed and PWM Duty Cycle section.
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
RANGE 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
1 0°C
13.3°
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
C
Example: Determining T
Channel
The following example shows how the different T
settings can be applied to three different thermal zon
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 the Step 2:
Configuring the Mux section, 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 fro nt chassis fan and rear chassis fan connected to
P WM2 and PWM3. The front chassis fan is configured to run
at PWM = 20%. The rear chassis fan is configured torun at
MIN
PWM = 30%. The CPU fan is configured to run at
MIN
PWM = 10%.
MIN
Rev. A | Page 43 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 Temperat
RANGE
values apply:
RANGE
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
ure
an d T
MIN RANGE
es. In this