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ADT7473
STEP 4: PWM
FOR EACH PWM (FAN) OUTPUT
MIN
PWM
is the minimum PWM duty cycle at which each fan in
MIN
the system runs. It is also the start speed for each fan under
automatic fan control once the temperature rises above T
Refer to Figure 50. For maximum system acoustic benefit,
PWM
should be set as low as possible. Depending on the fan
MIN
used, the PWM
setting is usually in the 20% to 33% duty
MIN
cycle range. This value can be found through fan validation.
100%
PWM
MIN
0%
T
MIN
Figure 50. PWM
Determines Minimum PWM Duty Cycle
MIN
More than one PWM output can be controlled from a single
temperature measurement channel. For example, Remote 1
temperature can control PWM1 and PWM2 outputs. If two
different fans are used on PWM1 and PWM2, the fan
characteristics can be set up differently. As a result, Fan 1 driven
by PWM1 can have a different PWM
connected to PWM2. Figure 51 illustrates this as PWM1
(front fan) is turned on at a minimum duty cycle of 20%, while
PWM2
(rear fan) turns on at a minimum of 40% duty cycle.
MIN
However, both fans turn on at exactly the same temperature,
defined by T
.
MIN
100%
PWM2
MIN
PWM1
MIN
0%
T
MIN
Figure 51. Operating Two Different Fans
from a Single Temperature Channel
.
MIN
TEMPERATURE
value than that of Fan 2
MIN
MIN
TEMPERATURE
Rev. 0 | Page 38 of 76
Programming the PWM
MIN
The PWM
registers are 8-bit registers that allow the
MIN
minimum PWM duty cycle for each output to be configured
anywhere from 0% to 100%. This allows the minimum PWM
duty cycle to be set in steps of 0.39%.
The value to be programmed into the PWM
Value (decimal) = PWM
Example 1: For a minimum PWM duty cycle of 50%,
Value (decimal) = 50/0.39 = 128 (decimal)
Value = 128 (decimal) or 80 (hex)
Example 2: For a minimum PWM duty cycle of 33%,
Value (decimal) = 33/0.39 = 85 (decimal)
Value = 85 (decimal) or 54 (hex)
PWM
Registers
MIN
Reg. 0x64, PWM1 Minimum Duty Cycle = 0x80 (50% default)
Reg. 0x65, PWM2 Minimum Duty Cycle = 0x80 (50% default)
Reg. 0x66, PWM3 Minimum Duty Cycle = 0x80 (50% default)
Note on Fan Speed and PWM Duty Cycle
The PWM duty cycle does not directly correlate to fan speed in
RPM. Running a fan at 33% PWM duty cycle does not equate to
running the fan at 33% speed. Driving a fan at 33% PWM duty
cycle actually runs the fan at closer to 50% of its full speed. This
is because fan speed in %RPM generally relates to the square
root of PWM duty cycle. Given a PWM square wave as the
drive signal, fan speed in RPM approximates to
=
%
fanspeed
PWM
duty
STEP 5: PWM
FOR PWM (FAN) OUTPUTS
MAX
PWM
is the maximum duty cycle that each fan in the system
MAX
runs at under the automatic fan speed control loop. For
maximum system acoustic benefit, PWM
possible, but should be capable of maintaining the processor
temperature limit at an acceptable level. If the THERM
temperature limit is exceeded, the fans are still boosted to 100%
for fail-safe cooling. Refer to Figure 52.
There is a PWM
limit for each fan channel. The default value
MAX
of this register is 0xFF and thus has no effect unless it is
programmed.
Registers
register is given by
MIN
/0.39
MIN
×
cycle
10
should be as low as
MAX
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