參數(shù)資料
型號(hào): AMD1027
廠商: Analog Devices, Inc.
英文描述: dBCOOL Remote Thermal Controller and Voltage Monitor
中文描述: dBCOOL遠(yuǎn)程熱控制器和電壓監(jiān)視器
文件頁(yè)數(shù): 31/56頁(yè)
文件大?。?/td> 1072K
代理商: AMD1027
REV. A
ADM1027
–31–
Step 4
Determine PWM
MIN
for Each PWM (Fan) Output
PWM
MIN
is the minimum PWM duty cycle that each fan in the
system will run at. 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. Starting the fans at higher speeds than necessary
will merely make the system louder than needed. Depending on
the fan used, the PWM
MIN
setting should be in the range 20%
to 33% duty cycle. This value can be found through
fan validation.
TEMPERATURE
T
MIN
100%
PWM
MIN
0%
P
Figure 30. PWM
MIN
Determines Minimum PWM Duty Cycle
It is important to note that 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,
then the fan characteristics can be set up differently. So Fan 1
driven by PWM1 can have a different PWM
MIN
value than
that of Fan 2 connected to PWM2. Figure 31 illustrates this as
PWM1
MIN
(front fan) is turned on at a minimum duty cycle of
20%, whereas PWM2
MIN
(rear fan) turns on at a minimum of
40% duty cycle. Note however, that both fans turn on at the
exact same temperature, defined by T
MIN
.
TEMPERATURE
T
MIN
100%
PWM1
MIN
0%
P
PWM1
PWM2
PWM2
MIN
Figure 31. Operating Two Different Fans from a
Single-Temperature Channel
Programming the PWM
MIN
Registers
The PWM
MIN
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 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
Example 1:
For a minimum PWM Duty Cycle of 50%,
Value (decimal)
= 50/0.39 = 128
decimal
Value
= 128
decimal
or 80
hex
PWM
MIN
)
=
0 39
.
Example 2:
For a minimum PWM duty cycle of 33%,
Value (decimal)
= 33/0.39 = 85
decimal
Value
= 85
decimal
or 54
hex
PWM
MIN
Registers
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)
Fan Speed and PWM Duty Cycle
Note that 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 relates to the square
root of PWM duty cycle. Given a PWM square wave as the
drive signal, fan speed in RPM equates to
%
fan speed
PWM duty cycle
=
10
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