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ADM1031
140
120
selected and the ADC sample rate programmed into the fan
120
100
filter register.
90
90
100
80
60
40
20
R TEMP
PWM DUTY CYCLE
80
60
40
20
80
70
60
50
40
30
PWM DUTY CYCLE
R TEMP
80
70
60
50
40
30
0
0
TIME (s)
54
0
20
10
20
10
Figure 31. Filtered Mode with Ramp Rate = 2
0
0
Finally, Figure 32 shows how the control loop reacts to
temperature with the slowest ramp rate. The ramp rate is set
to 1, while all other control parameters remain the same.
With the slowest ramp rate selected, it took 112 seconds for
the fan to reach full speed.
TIME (s)
Figure 33. How Fan Reacts to Temperature Variation
in Filtered Mode
Effect of ADC Sample Rate on Filtered Mode
120
140
The second way to change the filtered mode
characteristics is to adjust the ADC sample rate. The faster
100
80
60
40
20
R TEMP
PWM DUTY CYCLE
120
100
80
60
40
20
the ADC sample rate, the more temperature samples are
obtained per second. One way to apply filtering to the
control loop is to slow down the ADC sampling rate. This
means that the number of iterations of the filtered mode
algorithm per second is effectively reduced. If the number
of temperature measurements per second is reduced, how
often the PWM_OUT signal controlling the fan is updated
is also reduced.
Bits <4:2> of the fan filter register (Register 0 ? 23) set the
ADC sample rate. The default ADC sample rate is 1.4 kHz.
0
0
TIME (s)
112
0
The ADC sample rate is selectable from 87.5 Hz to
11.2 kHz. Table 13 shows how many temperature samples
are obtained per second, for each of the ADC sample rates.
Figure 32. Filtered Mode with Ramp Rate = 1
As can be seen from Figure 29 through Figure 32, the rate
at which the fan reacts to temperature change is dependent
on the ramp rate selected in the fan filter register. The higher
the ramp rate, the faster the fan reaches the newly calculated
Table 13. TEMPERATURE UPDATES PER SECOND
ADC Sample Rate Temperature Updates/Sec
87.5 Hz 0.0625
175 Hz 0.125
fan speed.
Figure 33 shows the behavior of the PWM output as
temperature varies. As the temperature rises, the fan speed
ramps up. Small drops in temperature do not affect the
ramp-up function because the newly calculated fan speed is
still higher than the previous PWM value. The filtered mode
allows the PWM output to be made less sensitive to
temperature variations. This is dependent on the ramp rate
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19
350 Hz
700 kHz
1.4 kHz
2.8 kHz
5.6 kHz
11.2 kHz
0.25
0.5
1 (Default)
2
4
8
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