adt7476a Analog Devices, Inc., adt7476a Datasheet - Page 43

• Manufacturer Part Number: adt7476a
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

STEP 4—PWM
PWM
in 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
ting 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, 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 55 illustrates this as PWM1
(front fan), which is turned on at a minimum duty cycle of 20%,
while PWM2
cycle. Note: Both fans turn on at exactly the same temperature,
defined by T
Programming the PWM Minimum Duty Cycle Registers
The PWM minimum duty cycle 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 the
minimum PWM duty cycle to be set in steps of 0.39%.
Figure 55. Operating Two Different Fans from a Single Temperature Channel
MIN
PWM2
PWM1
PWM
Figure 54. PWM
is the minimum PWM duty cycle at which each fan
100%
100%
MIN
0%
MIN
MIN
0%
MIN
MIN
.
(rear fan) turns on at a minimum of 40% duty
MIN
MIN
FOR EACH PWM (FAN) OUTPUT
T
T
MIN
Determines Minimum PWM Duty Cycle
MIN
TEMPERATURE
TEMPERATURE
MIN
value than that of Fan 2
MIN
should be as
MIN
MIN
MIN
set-
.
Rev. 0 | Page 43 of 72
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 Minimum Duty Cycle Registers
Register 0x64, PWM1 Minimum Duty Cycle = 0x80 (50% default)
Register 0x65, PWM2 Minimum Duty Cycle = 0x80 (50% default)
Register 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 maxi-
mum 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
value of this register is 0xFF and 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)
%
MAX
Fan
PWM
Figure 56. PWM
PWM
is the maximum duty cycle that each fan in the system
100%
MAX
Speed
MIN
0%
MAX
Below the THERM Temperature Limit
=
MAX
limit for each fan channel. The default
MAX
PWM
T
FOR PWM (FAN) OUTPUTS
MIN
Determines Maximum PWM Duty Cycle
Duty
MIN
/0.39
Cycle
MAX
TEMPERATURE
×
10
should be as low as
MIN
register is
ADT7476A