ADM1030
It should be noted however, that changing the minimum PWM
duty cycle affects the control loop behavior.
Slope 1 of Figure 10 shows T
MIN
set to 0°C and the T
RANGE
chosen is 40°C. In this case, the fan’s PWM duty cycle will vary
over the range 33% to 100%. The fan will run 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 Slopes 2 and 3. Take
Case 2, where the minimum PWM duty cycle is reprogrammed
from 33% (default) to 53%.
The temperature at which the fan will run full-speed (100%
duty cycle) is given by:
T
MAX
=
T
MIN
+ ((Max
DC
–
Min DC)
×
T
RANGE
/10)
where,
T
MAX
T
MIN
= Temperature at which fan runs full-speed.
= 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).
100
93
T
RANGE
T
MIN
= PWM Duty Cycle versus Temperature Slope.
= 0
°
C, T
RANGE
= 40
°
C
Example 1
40
C
PWM DUTY CYCLE – %
87
73
66
60
53
47
40
33
0
T
MIN
16
RA
NG
E
=
80
Min DC = 53% = 8 decimal (Table VII)
Calculate T
MAX
.
T
MAX
T
MAX
T
MAX
T
MAX
= T
MIN
+ ((Max DC – Min DC)
×
T
RANGE
/10)
= 0 + ((100% DC – 53% DC)
×
40/10)
= 0 + ((15 – 8)
×
4) = 28
=
28 C
(As seen on Slope 2 of Figure 10)
= 0
°
C, T
RANGE
= 40
°
C
T
Example 2
28
40
60
T
MIN
TEMPERATURE – C
Min DC
= 73% = 11 Decimal (Table VII)
Calculate T
MAX
.
T
MAX
T
MAX
T
MAX
T
MAX
T
MIN
= T
MIN
+ ((Max DC – Min DC)
×
T
RANGE
/10)
= 0 + ((100% DC – 73% DC)
×
40/10)
= 0 + ((15 – 11)
×
4) = 16
=
16 C
(As seen on Slope 3 of Figure 10)
= 0°C, T
RANGE
= 40°C
Figure 10. Effect of Changing Minimum Duty Cycle on
Control Loop with Fixed T
MIN
and T
RANGE
Values
The fan will actually reach full-speed at a much lower tempera-
ture, 28°C. 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°C. So the effect of increasing the mini-
mum 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 VII shows the relation-
ship between the decimal values written to the Fan Speed Config
Register and PWM duty cycle obtained.
Table VII. Programming PWM Duty Cycle
Example 3
Min DC
= 33% = 5 Decimal (Table VII)
Calculate T
MAX
.
T
MAX
T
MAX
T
MAX
T
MAX
= T
MIN
+ ((Max DC – Min DC)
×
T
RANGE
/10)
= 0 + ((100% DC – 33% DC)
×
40/10)
= 0 + ((15 – 5)
×
4) = 40
=
40 C
(As seen on Slope 1 of Figure 10)
Decimal Value
00
01
02
03
04
05
06
07
08
09
10 (0x0A)
11 (0x0B)
12 (0x0C)
13 (0x0D)
14 (0x0E)
15 (0x0F)
PWM Duty Cycle
0%
7%
14%
20%
27%
33% (Default)
40%
47%
53%
60%
67%
73%
80%
87%
93%
100%
In this case, since the Minimum Duty Cycle is the default 33%,
the equation for T
MAX
reduces to:
T
MAX
T
MAX
T
MAX
T
MAX
= T
MIN
+ ((Max DC – Min DC)
×
T
RANGE
/10)
= T
MIN
+ ((15 – 5)
×
T
RANGE
/10)
= T
MIN
+ (10
×
T
RANGE
/10)
=
T
MIN
+ T
RANGE
–14–
REV. 0
AD [ ANALOG DEVICES ]
