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ADM1031
READ
TEMPERATURE
CALCULATE
NEW PWM
DUTY CYCLE
Figure 29 shows remote temperature plotted against
PWM duty cycle for filtered mode. The ADC sample rate is
the highest sample rate; 11.25 kHz. The ramp rate is set to
8, which would correspond to the fastest ramp rate. With
these settings, it took approximately 12 seconds to go from
0% duty cycle to 100% duty cycle (full-speed). The T MIN
value = 32 ? C and the T RANGE = 80 ? C. Even though the
temperature increased very rapidly, the fan gradually ramps
up to full speed.
IS NEW
PWM VALUE >
PREVIOUS
VAULE?
YES
INCREMENT
PREVIOUS PWM
VALUE BY RAMP
RATE
NO
DECREMENT
PREVIOUS
PWM VALUE
BY RAMP RATE
140
120
100
80
60
40
R TEMP
PWM DUTY CYCLE
120
100
80
60
40
Figure 28. Filtered Mode Algorithm
20
20
The filtered mode algorithm calculates a new PWM duty
cycle based on the temperature measured. If the new PWM
duty cycle value is greater than the previous PWM value, the
0
0
TIME (s)
12
0
previous PWM duty cycle value is incremented by either 1,
2, 4, or 8 time slots (depending on the setting of bits <6:5>
of the fan filter register). If the new PWM duty cycle value
is less than the previous PWM value, the previous PWM
duty cycle is decremented by 1, 2, 4, or 8 time slots. Each
time the PWM duty cycle is incremented or decremented, it
is stored as the previous PWM duty cycle for the next
Figure 29. Filtered Mode with Ramp Rate = 8
Figure 30 shows how changing the ramp rate from 8 to 4
affects the control loop. The overall response of the fan is
slower. Because the ramp rate is reduced, it takes longer for
the fan to achieve full running speed. In this case, it took
approximately 22 seconds for the fan to reach full speed.
comparison.
What does an increase of 1, 2, 4, or 8 time slots
actually mean in terms of PWM duty cycle?
120
100
140
120
A ramp rate of 1 corresponds to one time slot, which is
1/240 of the PWM period. In filtered auto fan speed control
mode, incrementing or decrementing by 1 changes the PWM
output duty cycle by 0.416%.
Table 12. EFFECT OF RAMP RATES ON PWM_OUT
Ramp Rate PWM Duty Cycle Change
80
60
40
R TEMP
PWM DUTY CYCLE
100
80
60
40
1
0.416%
20
20
2
4
8
0.833%
1.66%
3.33%
0
0
TIME (s)
22
0
So programming a ramp rate of 1, 2, 4, or 8 simply
increases or decreases the PWM duty cycle by the amounts
shown in Table 12, depending on whether the temperature
is increasing or decreasing.
Figure 30. Filtered Mode with Ramp Rate = 4
Figure 31 shows the PWM output response for a ramp rate
of 2. In this instance the fan took about 54 seconds to reach
full running speed.
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