ADT7476 Datasheet, PDF (40/67 Pages) Analog Devices – dBCool Remote Thermal Controller and Voltage Monitor

ADT7476
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)
PWM Maximum Duty Cycle Registers
Register 0x38, PWM1 Maximum Duty Cycle = 0xFF
(100% default)
Register 0x39, PWM2 Maximum Duty Cycle = 0xFF
(100% default)
Register 0x3A, PWM3 Maximum Duty Cycle = 0xFF
(100% default)
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.
T_RANGE
100%
3. Determine the slope of the required control loop to
meet these requirements.
4. Using the ADT7476 evaluation software, you can
graphically program and visualize this
functionality.
As PWM_MIN is changed, the automatic fan control slope
changes.
100%
50%
33%
0%
30°C
T_MIN
Figure 58. Adjusting PWM_MIN Changes the Automatic
Fan Control Slope
As T_RANGE is changed, the slope changes. As T_RANGE
gets smaller, the fans reach 100% speed with a smaller
temperature change.
100%
10%
0%
T_{MIN–HYST}
T_MIN
30°C
40°C
45°C
54°C
Figure 59. Increasing T_RANGE Changes the
AFC Slope
PWM_MIN
0%
T_MIN
TEMPERATURE
Figure 57. T_RANGE Parameter Affects Cooling Slope
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.
100%
MAX
PWM
10%
0%
T_{MIN–HYST}
T_RANGE
Figure 60. Changing PWM Max Does Not Change the
AFC Slope
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