ADM1030
FILTERED CONTROL MODE
The Automatic Fan Speed Control Loop reacts instantaneously
to changes in temperature, i.e., the PWM duty cycle will respond
immediately to temperature change. In certain circumstances,
we may not want the PWM output to react instantaneously to
temperature changes. If significant variations in temperature
were found in a system, it would have the effect of changing the
fan speed, which could be obvious to someone in close proxim-
ity. One way to improve the system’s acoustics would be to
slow down the loop so that the fan ramps slowly to its newly
calculated fan speed. This also ensures that temperature transients
will effectively be ignored, and the fan’s operation will be smooth.
There are two means by which to apply filtering to the Auto-
matic Fan Speed Control Loop. The first method is to ramp the
fan speed at a predetermined rate, to its newly calculated value
instead of jumping directly to the new fan speed. The second
approach involves changing the on-chip ADC sample rate, to
change the number of temperature readings taken per second.
The filtered mode on the ADM1030 is invoked by setting Bit 0
of the Fan Filter Register (Register 0x23). Once the Fan Filter
Register has been written to, and all other control loop param-
eters (T
MIN
, T
RANGE
, etc.) have been programmed, the device
may be placed into Automatic Fan Speed Control Mode by
setting Bit 7 of Configuration Register 1 (Register 0x00) to 1.
Effect of Ramp Rate on Filtered Mode
READ
TEMPERATURE
CALCULATE
NEW PWM
DUTY CYCLE
IS NEW
PWM VALUE >
PREVIOUS
VALUE?
YES
NO
DECREMENT
PREVIOUS
PWM VALUE
BY RAMP
RATE
INCREMENT
PREVIOUS PWM
VALUE BY RAMP
RATE
Figure 12. Filtered Mode Algorithm
Bits <6:5> of the Fan Filter Register determine the ramp rate in
Filtered Mode. The PWM_OUT signal driving the fan will have
a period, T, given by the PWM_OUT drive frequency, f, since
T = 1/f. For a given PWM period, T, the PWM period is subdi-
vided into 240 equal time slots. One time slot corresponds to
the smallest possible increment in PWM duty cycle. A PWM
signal of 33% duty cycle will thus be high for 1/3
×
240 time
slots and low for 2/3
×
240 time slots. Therefore, 33% PWM
duty cycle corresponds to a signal which is high for 80 time slots
and low for 160 time slots.
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 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 Regis-
ter). 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 comparison.
So what does an increase of 1, 2, 4, or 8 time slots actually mean
in terms of PWM duty cycle?
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 VIII. Effect of Ramp Rates on PWM_OUT
PWM_OUT
33% DUTY
CYCLE
80 TIME
SLOTS
160 TIME
SLOTS
PWM OUTPUT
(ONE PERIOD) =
240 TIME SLOTS
Ramp Rate
1
2
4
8
PWM Duty Cycle Change
0.416%
0.833%
1.66%
3.33%
Figure 11. 33% PWM Duty Cycle Represented in Time
Slots
The ramp rates in Filtered Mode are selectable between 1, 2, 4,
and 8. The ramp rates are actually discrete time slots. For
example, if the ramp rate = 8, then eight time slots will be added
to the PWM_OUT high duty cycle each time the PWM_OUT
duty cycle needs to be increased. Figure 12 shows how the
Filtered Mode algorithm operates.
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 V, depending on whether the temperature is increasing
or decreasing.
Figure 13 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. It can be seen that even though the temperature increased
very rapidly, the fan gradually ramps up to full speed.
–16–
REV. 0
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