Step 4: Pwm Min For Each Pwm (Fan) Output; Step 5: Pwm Max For Pwm (Fan) Outputs - Analog Devices dBCool ADT7467 Manual

STEP 4: PWM FOR EACH PWM (FAN) OUTPUT
MIN
PWM is the minimum PWM duty cycle at which each fan in
MIN
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, PWM
low as possible. Depending on the fan used, the PWM
is usually in the 20% to 33% duty cycle range. This value can be
found through fan validation.
100%
PWM
MIN
0%
T
MIN
Figure 56. PWM Determines Minimum PWM Duty Cycle
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, then the fan
characteristics can be set up differently. As a result, Fan 1 driven
by PWM1 can have a different PWM
connected to PWM2. Figure 57 illustrates this as PWM1
(front fan) is turned on at a minimum duty cycle of 20%, while
PWM2 (rear fan) turns on at a minimum of 40% duty cycle.
MIN
Note, however, that both fans turn on at exactly the same
temperature, defined by T
MIN
100%
PWM2
MIN
PWM1
MIN
0%
T
MIN
Figure 57. Operating Two Different Fans from a Single Temperature Channel
should be as
MIN
MIN
TEMPERATURE
value than that of Fan 2
MIN
MIN
.
TEMPERATURE
Programming the PWM
The PWM
MIN
minimum PWM duty cycle for each output to be configured
anywhere from 0% to 100%. This allows the minimum PWM
.
MIN
duty cycle to be set in steps of 0.39%.
setting
The value to be programmed into the PWM
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)
PWM Registers
MIN
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)
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
STEP 5: PWM
PWM is the maximum duty cycle that each fan in the system
MAX
runs at under the automatic fan speed control loop. For
maximum system acoustic benefit, PWM
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
of this register is 0xFF and so has no effect unless it is
programmed.
Rev. 0| Page 41 of 80
Registers
MIN
registers are 8-bit registers that allow the
/0.39
MIN
= ×
PWM duty cycle 10
FOR PWM (FAN) OUTPUTS
MAX
MAX
limit for each fan channel. The default value
MAX
ADT7467
register is given
MIN
should be as low as