ADT7468 Analog Devices, Inc., ADT7468 Datasheet - Page 46

• Manufacturer Part Number: ADT7468
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

ADT7468
STEP 4: PWM
PWM
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
setting is usually in the 20% to 33% duty cycle range. This value
can be found through fan validation.
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 60 illustrates this as PWM1
(front fan) is turned on at a minimum duty cycle of 20%, while
PWM2
Note, however, that both fans turn on at exactly the same
temperature, defined by T
Figure 60. Operating Two Different Fans from a Single Temperature Channel
MIN
MIN
PWM2
PWM1
PWM
Figure 59. PWM
is the minimum PWM duty cycle at which each fan in
100%
100%
(rear fan) turns on at a minimum of 40% duty cycle.
MIN
0%
MIN
MIN
0%
MIN
FOR EACH PWM (FAN) OUTPUT
MIN
T
T
MIN
Determines Minimum PWM Duty Cycle
MIN
MIN
.
TEMPERATURE
TEMPERATURE
MIN
value than that of Fan 2
MIN
should be as
MIN
MIN
MIN
.
Rev. A | Page 46 of 84
Programming the PWM
The PWM
mum 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%.
The value to be programmed into the PWM
given by
Example 1: For a minimum PWM duty cycle of 50%,
Example 2: For a minimum PWM duty cycle of 33%,
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)
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
STEP 5: PWM
PWM
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.
Value (decimal) = PWM
Value (decimal) = 50/0.39 = 128 (decimal)
Value = 128 (decimal) or 80 (hex)
Value (decimal) = 33/0.39 = 85 (decimal)
Value = 85 (decimal)l or 54 (hex)
%
MIN
MAX
fanspeed
Registers
is the maximum duty cycle that each fan in the system
MIN
registers are 8-bit registers that allow the mini-
MAX
MAX
=
limit for each fan channel. The default value
PWM
FOR PWM (FAN) OUTPUTS
MIN
duty
Registers
MIN
/0.39
cycle
×
10
MAX
MIN
should be as low as
register is