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Step 4 − PWM
MIN
PWM
is the minimum PWM duty cycle at which each
MIN
fan in the system runs. It is also the start speed for each fan
under automatic fan control once the temperature rises
above T
. For maximum system acoustic benefit,
MIN
PWM
should be as low as possible. Depending on the
MIN
fan used, the PWM
duty cycle range. This value can be found through fan
validation.
100%
PWM
MIN
0%
Figure 54. PWM
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
than that of Fan 2 connected to PWM2. Figure 55 illustrates
this as PWM1
minimum duty cycle of 20%, while PWM2
turns on at a minimum of 40% duty cycle.
NOTE: Both fans turn on at exactly the same temperature, defined
by T
.
MIN
100%
PWM2
MIN
PWM1
MIN
0%
Figure 55. Operating Two Different Fans from
a Single Temperature Channel
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%.
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for Each PWM (Fan) Output
setting is usually in the 20% to 33%
MIN
TEMPERATURE
T
MIN
Determines Minimum
MIN
PWM Duty Cycle
(front fan), which is turned on at a
MIN
PWM2
PWM1
TEMPERATURE
T
MIN
ADT7476A
The value to be programmed into the PWM
given by:
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) l or 54 (hex)
Table 45. PWM MINIMUM DUTY CYCLE REGISTERS
Register
0x64
0x65
0x66
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.
value
MIN
Given a PWM square wave as the drive signal, fan speed in
RPM approximates to:
(rear fan)
MIN
Step 5 − PWM
PWM
MAX
system runs at under the automatic fan speed control loop.
For maximum system acoustic benefit, PWM
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
default value of this register is 0xFF and has no effect unless
it is programmed.
PWM
PWM
Figure 56. PWM
Cycle Below the THERM Temperature Limit
www.onsemi.com
41
/0.39
MIN
Description
PWM1 Minimum Duty Cycle
PWM2 Minimum Duty Cycle
PWM3 Minimum Duty Cycle
% fanspeed + PWM Duty Cycle
for PWM (Fan) Outputs
MAX
is the maximum duty cycle that each fan in the
limit for each fan channel. The
MAX
100%
MAX
MIN
0%
TEMPERATURE
T
MIN
Determines Maximum PWM Duty
MAX
register is
MIN
Default
0x80 (50%)
0x80 (50%)
0x80 (50%)
(eq. 5)
10
should be
MAX
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