ADT7476A
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Step 4 − PWM
MIN
for Each PWM (Fan) Output
PWM
MIN
is the minimum PWM duty cycle at which each
fan in the system runs. It is also the start speed for each fan
under automatic fan control once the temperature rises
above T
MIN
. For maximum system acoustic benefit,
PWM
MIN
should be as low as possible. Depending on the
fan used, the PWM
MIN
setting is usually in the 20% to 33%
duty cycle range. This value can be found through fan
validation.
Figure 54. PWM
MIN
Determines Minimum
PWM Duty Cycle
T
MIN
100%
0%
PWM
DUTY CYCLE
TEMPERATURE
PWM
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
MIN
value
than that of Fan 2 connected to PWM2. Figure 55 illustrates
this as PWM1
MIN
(front fan), which is turned on at a
minimum duty cycle of 20%, while PWM2
MIN
(rear fan)
turns on at a minimum of 40% duty cycle.
NOTE: Both fans turn on at exactly the same temperature, defined
by T
MIN
.
Figure 55. Operating Two Different Fans from
a Single Temperature Channel
T
MIN
100%
0%
TEMPERATURE
PWM1
MIN
PWM2
MIN
PWM1
PWM2
PWM
DUTY CYCLE
Programming the PWM Minimum Duty Cycle Registers
The PWM minimum duty cycle registers are 8-bit
registers that allow the 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
MIN
register is
given by:
Value (decimal) = PWM
MIN
/0.39
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) l or 54 (hex)
Table 45. PWM MINIMUM DUTY CYCLE REGISTERS
Register
Description
Default
0x64
PWM1 Minimum Duty Cycle
0x80 (50%)
0x65
PWM2 Minimum Duty Cycle
0x80 (50%)
0x66
PWM3 Minimum Duty Cycle
0x80 (50%)
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 Cycle
10
Ǹ
(eq. 5)
Step 5 − PWM
MAX
for PWM (Fan) Outputs
PWM
MAX
is the maximum duty cycle that each fan in the
system runs at under the automatic fan speed control loop.
For maximum system acoustic benefit, PWM
MAX
should be
as low as 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.
There is a PWM
MAX
limit for each fan channel. The
default value of this register is 0xFF and has no effect unless
it is programmed.
Figure 56. PWM
MAX
Determines Maximum PWM Duty
Cycle Below the THERM Temperature Limit
T
MIN
100%
0%
TEMPERATURE
PWM
MIN
PWM
MAX
PWM
DUTY CYCLE
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