ADT7476A
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42
Programming the PWM Maximum Duty Cycle
Registers
The PWM maximum duty cycle registers are 8-bit
registers that allow the maximum PWM duty cycle for each
output to be configured anywhere from 0% to 100%. This
allows the maximum PWM duty cycle to be set in steps of
0.39%.
The value to be programmed into the PWM maximum
duty cycle register is given by:
Value (decimal) = PWM
MAX
/0.39
Example 1:
For a maximum 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 75%,
Value (decimal) = 75/0.39 = 85 (decimal)
Value = 192 (decimal) or C0 (hex)
Table 46. PWM MAXIMUM DUTY CYCLE REGISTERS
Register
Description
Default
0x38
PWM1 Maximum Duty Cycle
0xFF (100%)
0x39
PWM2 Maximum Duty Cycle
0xFF (100%)
0x3A
PWM3 Maximum Duty Cycle
0xFF (100%)
Step 6 − T
RANGE
for Temperature Channels
T
RANGE
is the range of temperature over which automatic
fan control occurs once the programmed T
MIN
temperature
has been exceeded. T
RANGE
is the temperature range between
PWM
MIN
and 100% PWM where the fan speed changes
linearly. Otherwise stated, it is the line drawn between the
T
MIN
/PWM
MIN
and the (T
MIN
+ T
RANGE
)/PWM100%
intersection points.
Figure 57. T
RANGE
Parameter Affects Cooling Slope
T
MIN
100%
0%
TEMPERATURE
PWM
MIN
PWM
DUTY CYCLE
T
RANGE
The T
RANGE
is determined by the following procedure:
1. Determine the maximum operating temperature for
that channel (for example, 70
°
C).
2. Determine experimentally the fan speed (PWM
duty cycle value) that does not exceed the
temperature at the worst-case operating points. For
example, 70
°
C is reached when the fans are
running at 50% PWM duty cycle.
3. Determine the slope of the required control loop to
meet these requirements.
4. Using the ADT7476A evaluation software, you
can graphically program and visualize this
functionality.
As PWM
MIN
is changed, the automatic fan control slope
changes.
Figure 58. Adjusting PWM
MIN
Changes the Automatic
Fan Control Slope
T
MIN
100%
0%
30
°
C
PWM
DUTY CYCLE
33%
50%
As T
RANGE
is changed, the slope changes. As T
RANGE
gets smaller, the fans reach 100% speed with a smaller
temperature change.
Figure 59. Increasing T
RANGE
Changes the AFC Slope
T
MIN
100%
0%
30
°
C
PWM
DUTY CYCLE
10%
40
°
C
45
°
C
54
°
C
T
MIN−HYST
Figure 60. Changing PWM
MAX
Does Not Change the
AFC Slope
T
RANGE
100%
0%
PWM
DUTY CYCLE
10%
T
MIN−HYST
MAX
PWM
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