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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
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
0x38
PWM1 Maximum Duty Cycle
0x39
PWM2 Maximum Duty Cycle
0x3A
PWM3 Maximum Duty Cycle
Step 6 − T
RANGE
T
is the range of temperature over which automatic
RANGE
fan control occurs once the programmed T
has been exceeded. T
PWM
and 100% PWM where the fan speed changes
MIN
linearly. Otherwise stated, it is the line drawn between the
T
/PWM
MIN
MIN
intersection points.
100%
PWM
MIN
0%
Figure 57. T
RANGE
The T
is determined by the following procedure:
RANGE
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
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/0.39
MAX
Description
Default
0xFF (100%)
0xFF (100%)
0xFF (100%)
for Temperature Channels
MIN
is the temperature range between
RANGE
and the (T
+ T
)/PWM100%
MIN
RANGE
T
RANGE
TEMPERATURE
T
MIN
Parameter Affects Cooling Slope
ADT7476A
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
changes.
100%
50%
33%
0%
Figure 58. Adjusting PWM
As T
RANGE
gets smaller, the fans reach 100% speed with a smaller
temperature change.
100%
temperature
10%
0%
Figure 59. Increasing T
100%
MAX
PWM
10%
0%
Figure 60. Changing PWM
www.onsemi.com
42
is changed, the automatic fan control slope
MIN
30°C
T
MIN
Changes the Automatic
MIN
Fan Control Slope
is changed, the slope changes. As T
30°C
T
40°C
MIN−HYST
45°C
54°C
T
MIN
Changes the AFC Slope
RANGE
T
RANGE
T
MIN−HYST
Does Not Change the
MAX
AFC Slope
RANGE
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