Step 4: Pwm Min For Each Pwm (Fan) Output; Max - Analog Devices dBCool ADT7473 Manual

ADT7473
STEP 4: PWM FOR EACH PWM (FAN) OUTPUT
MIN
PWM is the minimum PWM duty cycle at which each fan
MIN
the system runs. It is also the start speed for each fan under
automatic fan control once the temperature rises above T
(see Figure 50). For maximum system acoustic benefit, PWM
should be set as low a s possible. Depending on the fan used, the
P WM setting is usually in the 20% to 33% duty cycle range.
MIN
This va lue can be found through fan validation.
100%
PWM
MIN
0%
T
MIN
Figure 50. PWM Determines Minimum PWM Duty Cycle
MIN
More tha n one PWM output can e controlled from si
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 s et up differently. As a result, Fan 1 driven
by PWM1 can have a different PWM
conn ected to PWM2. Figure 51 illustrates this as PWM
(front fa n) 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 temperatur
defined by T .
MIN
100%
PWM2
MIN
PWM1
MIN
0%
T
MIN
Figure 51. Operating Two Different Fans
from a Single Temperature Channel
MIN
MIN
TEMPERATURE
b a ngle
value than that of Fan 2
MIN
1
MIN
e,
TEMPERATURE
Rev. A | Page 40 of 76
Programming the PWM
The PWM registers are 8-bit registers that allow the
in
MIN
min im um PW M
anyw here from 0% to 100%. This allows the minimum
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)
Exam ple 2: For a minimum PWM duty cy
Value (decimal) = 33/0.39 = 85 (decimal)
Value = 85 (decimal) or 54 (hex)
PWM Registers
MIN
Register 0x64, PWM1
(50% default)
Register 0x 65, PWM2
(50% d efault)
Regist er 0x66, PWM3
(50% d efault)
Note on Fan Speed a
The P WM duty cycle d
RPM. Running a fan a
runnin g the fan at 33%
cycle a ctually runs the fan at closer to 50% of its full speed. This
is beca use fan speed in
root o f PWM duty cycl
drive s ignal, fan speed
% fanspeed
STEP 5: PWM
PWM is the maximum duty cycle at w

MAX

system runs under the automatic fa
maximum system acoustic benefit, PWM
possible, but should be capable of maintaining the process
temperature limit at an acceptable level. If the THERM
temperature limi
for f ail-safe cooling (see Figure 52).
There is a PWM
of this register is 0
programmed.
Registers
MIN
duty cycle for each output t
MIN
/0.39
MIN
cle of 33%
Minimum Duty Cycle = 0x80
Minimum D uty Cycle = 0x80
Minim um Duty Cycle = 0x80
nd P WM Duty Cycle
oes n ot directly correlate to fan speed in
t 33% P WM duty cycle does not equate to
speed . Driving a fan at 33% PWM duty
%R PM generally relates to the square
e. Give n a PWM square wave as the
in RPM appr oximates to
= ×
PW M du ty cycle 10
FOR PW (FAN) OUTPUTS M
MAX
hich each fan in the
n speed control loop. For
MAX
t is exceeded, the fans are still boosted to 100%
limit for each fan channel. The default
MAX
xFF and thus has no effect unless it is
o be config ured
PWM
register is given by
should b e as low as
or
value