ADM1027ARQZ-RL71 ONSEMI [ON Semiconductor], ADM1027ARQZ-RL71 Datasheet - Page 31

• Manufacturer Part Number: ADM1027ARQZ-RL71
• Manufacturer: ONSEMI [ON Semiconductor]

Step 4
Determine PWM
PWM
system will run at. It is also the start speed for each fan under
automatic fan control once the temperature rises above T
For maximum system acoustic benefit, PWM
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
to 33% duty cycle. This value can be found through
fan validation.
Figure 30. PWM
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
that of Fan 2 connected to PWM2. Figure 31 illustrates this as
PWM1
20%, whereas PWM2
40% duty cycle. Note however, that both fans turn on at the
exact same temperature, defined by T
REV. A
MIN
Figure 31. Operating Two Different Fans from a
Single-Temperature Channel
MIN
PWM2
PWM1
PWM
is the minimum PWM duty cycle that each fan in the
(front fan) is turned on at a minimum duty cycle of
100%
100%
MIN
0%
MIN
MIN
0%
MIN
MIN
for Each PWM (Fan) Output
Determines Minimum PWM Duty Cycle
MIN
MIN
T
T
MIN
(rear fan) turns on at a minimum of
MIN
setting should be in the range 20%
TEMPERATURE
TEMPERATURE
MIN
.
MIN
MIN
Rev. 3 | Page 31 of 56 | www.onsemi.com
should be as
value than
MIN
.
–31–
Programming the PWM
The PWM
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
given by
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
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
MIN
Registers
MIN
% fan speed
registers are 8-bit registers that allow the minimum
Value decimal
(
MIN
=
Registers
PWM duty cycle
)
=
PWM
MIN
ADM1027
0 39
MIN
.
¥ 10
register is