ADM1030
http://onsemi.com
15
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, i.e., which temperature channel
controls the fan.
6. Select Automatic Fan Speed Control Mode by
setting Bit 7 of Configuration Register 1.
Other Control Loop Parameters
Having programmed all the above loop parameters, are
there any other parameters to worry about?
T
MIN
was defined as being the temperature at which the
fan switched on and ran at minimum speed. This minimum
speed is 33% duty cycle by default. If the minimum PWM
duty cycle is programmed to 33%, the fan control loops will
operate as previously described.
It should be noted however, that changing the minimum
PWM duty cycle affects the control loop behavior.
Slope 1 of Figure 26 shows T
MIN
set to 0癈 and the
T
RANGE
chosen is 40癈. In this case, the fans PWM duty
cycle will vary over the range 33% to 100%. The fan will run
full-speed at 40癈. If the minimum PWM duty cycle at
which the fan runs at T
MIN
is changed, its effect can be seen
on Slopes 2 and 3. Take Case 2, where the minimum PWM
duty cycle is reprogrammed from 33% (default) to 53%.
33
40
53
60
73
80
87
93
100
Figure 26. Effect of Changing Minimum Duty Cycle
on Control Loop with Fixed T
MIN
and T
RANGE
Values
TEMPERATURE (癈)
T
MIN
0
16
28
60
1
2
3
40
T
RANGE
= 40癈
47
66
The fan will actually reach full-speed at a much lower
temperature, 28癈. Case 3 shows that when the minimum
PWM duty cycle was increased to 73%, the temperature at
which the fan ran full-speed was 16癈. So the effect of
increasing the minimum PWM duty cycle, with a fixed T
MIN
and fixed T
RANGE
, is that the fan will actually reach
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 Config Register (Register 0x22). Table 11 shows the
relationship between the decimal values written to the Fan
Speed Config Register and PWM duty cycle obtained.
Table 11. PROGRAMMING PWM DUTY CYCLE
Decimal Value
PWM Duty Cycle
00
0%
01
7%
02
14%
03
20%
04
27%
05
33% (Default)
06
40%
07
47%
08
53%
09
60%
10 (0x0A)
67%
11 (0x0B)
73%
12 (0x0C)
80%
13 (0x0D)
87%
14 (0x0E)
93%
15 (0x0F)
100%
The temperature at which the fan will run full-speed
(100% duty cycle) is given by:
(eq. 2)
T
MAX
+ T
MIN
) ((Max DC * Min DC) T
RAN E
10)
where:
T
MAX
= Temperature at which fan runs full speed
T
MIN
= Temperature at which fan will turn on
Max DC = Maximum Duty Cycle (100%) = 15 decimal
Min DC = Duty Cycle at T
MIN
, programmed into
Fan Speed Config Register
(default = 33% = 5 decimal)
T
RANGE
= PWM Duty Cycle versus Temperature Slope
Example 1
T
MIN
= 0?/SPAN>C, T
RANGE
= 40癈
Min DC = 53% = 8 decimal (Table 11)
Calculate T
MAX
(eq. 3)
T
MAX
+ T
MIN
) ((Max DC * Min DC) T
RANGE
10)
T
MAX
+ 0 ) ((100% DC * 53% DC) 4010)
T
+ 0 ) ((15 * 8) 4) + 28
T
MAX
=285C. (As seen on Slope 2 of Figure 26)
Example 2
T
MIN
= 0?/SPAN>C, T
RANGE
= 40癈
Min DC = 73% = 11 decimal (Table 11)
Calculate T
MAX
(eq. 4)
T
MAX
+ T
MIN
) ((Max DC * Min DC) T
RANGE
10)
T
MAX
+ 0 ) ((100% DC * 73% DC) 4010)
T
+ 0 ) ((15 * 11) 4) + 16
T
MAX
=165C. (As seen on Slope 3 of Figure 26)
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