ADM1031
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17
Register 024 Local Temperature T
MIN
/T
RANGE
<7:3> Local Temperature T
MIN
. These bits set the
temperature at which the fan turns on when
under auto fan speed control. T
MIN
can be
programmed in 4C increments.
00000 = 0C
00001 = 4C
00010 = 8C
00011 = 12C
|
|
01000 = 32C (Default)
|
|
11110 = 120C
11111 = 124C
<2:0> Local Temperature T
RANGE
. This nibble sets
the temperature range over which automatic
fan speed control takes place.
000 = 5C
001 = 10C
010 = 20C
011 = 40C
100 = 80C
Register 025, 026 Remote 1, 2 Temperature
T
MIN
/T
RANGE
<7:3> Remote Temperature T
MIN
. Sets the
temperature at which the fan switches on
based on Remote X Temperature Readings.
00000 = 0C
00001 = 4C
00010 = 8C
00011 = 12C
|
|
01100 = 48C
|
|
11110 = 120C
11111 = 124C
<2:0> Remote Temperature T
RANGE
. This nibble
sets the temperature range over which the fan
is controlled based on remote temperature
readings.
000 = 5C
001 = 10C
010 = 20C
011 = 40C
100 = 80C
Filtered Control Mode
The automatic fan speed control loop reacts
instantaneously to changes in temperature, that is, the PWM
duty cycle responds immediately to temperature change. In
certain circumstances, we do not want the PWM output to
react instantaneously to temperature changes. If significant
variations in temperature are found in a system, the fan
speed changes, which could be obvious to someone in close
proximity. 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 are effectively ignored, and the fan’s
operation is smooth.
There are two means by which to apply filtering to the
automatic 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 ADM1031 is invoked by setting
Bit 0 of the fan filter register (Register 023) for Fan 1 and
Bit 1 for Fan 2. Once the fan filter register has been written
to, and all other control loop parameters (such as T
MIN
,
T
RANGE
) have been programmed, the device can be placed
into automatic fan speed control mode by setting Bit 7 of
Configuration Register 1 (Register 000) to 1.
Effect of Ramp Rate on Filtered Mode
Bits <6:5> of the fan filter register determine the ramp
rate in filtered mode. The PWM_OUT signal driving the fan
has 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 subdivided in to 240 equal time slots. One time slot
corresponds to the smallest possible increment in PWM duty
cycle. A PWM signal of 33% duty cycle is thus high for
1/3 240 time slots and low for 2/3 240 time slots.
Therefore, 33% PWM duty cycle corresponds to a signal
that is high for 80 time slots and low for 160 time slots.
Figure 27. 33% PWM Duty Cycle Represented
in Time Slots
80 TIME
SLOTS
160 TIME
SLOTS
PWM_OUT
33% DUTY
CYCLE
PWM OUTPUT
(ONE PERIOD) =
240 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 are
added to the PWM_OUT high duty cycle each time the
PWM_OUT duty cycle needs to be increased. Figure 28
shows how the filtered mode algorithm operates.
