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ADM1031
100
93
87
80
The fan actually reaches full speed at a much lower
temperature, 28 ? C. Case 3 shows that when the minimum
PWM duty cycle is increased to 73%, the temperature at
which the fan runs full speed is 16 ? C. Therefore, the effect
of increasing the minimum PWM duty cycle, with a fixed
73
66
60
53
47
40
33
0
T MIN
T RANGE = 80 ? C
20 40 70 80
T MAX = T MIN + T RANGE
REMOTE TEMPERATURE ( ? C)
T MIN and fixed T RANGE , is that the fan actually reaches full
speed (T MAX ) at a lower temperature than T MIN + T RANGE .
How can T MAX be calculated?
In automatic fan speed control mode, the register that
holds the minimum PWM duty cycle at T MIN , is the fan
speed configuration register (Register 0 ? 22). Table 11
shows the relationship between the decimal values written
to the fan speed configuration register and PWM duty cycle
obtained.
Figure 25. Max Speed Calculated by Remote
Temperature Control Loop Drives Fan
Table 11. Programming PWM Duty Cycle
Decimal Value PWM Duty Cycle
Programming the Automatic Fan Speed Control
Loop
1. Program a value for T MIN.
2. Program a value for the slope T RANGE.
3. T MAX = T MIN + T RANGE.
4. Program a value for fan spin-up time.
5. Program the desired automatic fan speed control
mode behavior, that is, which temperature channel
controls the fan.
6. Select automatic fan speed control mode by setting
Bit 7 of Configuration Register 1.
00
01
02
03
04
05
06
07
08
09
0%
7%
14%
20%
27%
33% (Default)
40%
47%
53%
60%
Other Control Loop Parameters
It should be noted that changing the minimum PWM duty
cycle affects the control loop behavior.
Slope 1 of Figure 26 shows T MIN set to 0 ? C and the
T RANGE chosen is 40 ? C. In this case, the fan’s PWM duty
cycle varies over the range 33% to 100%. The fan runs
full-speed at 40 ? C. If the minimum PWM duty cycle at
which the fan runs at T MIN is changed, its effect can be seen
on Slope 2 and Slope 3. Take Case 2, where the minimum
PWM duty cycle is reprogrammed from 33% (default) to
10 (0 ? 0A) 67%
11 (0 ? 0B) 73%
12 (0 ? 0C) 80%
13 (0 ? 0D) 87%
14 (0 ? 0E) 93%
15 (0 ? 0F) 100%
The temperature at which the fan runs full-speed (100%
duty cycle) is given by:
53%.
T MAX + T MIN ) ((Max DC * Min DC)
T RANGE 10) (eq. 2)
100
93
87
80
73
66
3
2
where:
T MAX
T MIN
Max DC
Min DC
= Temperature at which fan runs full-speed
= Temperature at which fan will turn on
= Maximum Duty Cycle (100%) = 15 decimal
= Duty Cycle at T MIN , programmed into
Fan Speed Config Register
(default = 33% = 5 decimal)
60
53
47
40
33
1
T RANGE = 40 ? C
T RANGE = PWM Duty Cycle vs. Temperature Slope
Example 1 :
T MIN = 0 ? C, T RANGE = 40 ? C
Min DC = 53% = 8 decimal (Table 11)
0
T MIN
16 28 40
TEMPERATURE ( ? C)
60
Figure 26. Effect of Changing Minimum Duty Cycle
on Control Loop with Fixed T MIN and T RANGE Values
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