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ADT7490
100%
PWM2
as low as possible, but should be capable of maintaining the
processo r temperature limit at an acceptable level. If the
THERM temperature limit is exceeded, the fans are still
boosted to 100% for fail-safe cooling.
PWM2 MIN
PWM1 MIN
0%
T MIN
PWM1
TEMPERATURE
There is a PWM MAX limit for each fan channel. The
default value of this register is 0xFF and has no effect unless
it is programmed.
100%
PWM MAX
Figure 51. Operating Two Different Fans from
a Single Temperature Channel
Programming the PWM MIN Registers
The PWM MIN registers are 8-bit registers that allow the
minimum PWM duty cycle for each output to be configured
PWM MIN
0%
anywhere from 0% to 100%. This allows the minimum
PWM duty cycle to be set in steps of 0.39%.
T MIN
TEMPERATURE
The value to be programmed into the PWM MIN register is
given by:
Value (decimal) = PWM MIN /0.39%
Example 1
For a minimum PWM duty cycle of 50%,
Value (decimal) = 50%/0.39% = 128 (decimal)
Value = 128 (decimal) or 0x80 (hexadecimal)
Example 2
For a minimum PWM duty cycle of 33%,
Value (decimal) = 33%/0.39% = 85 (decimal)
Value = 85 (decimal) or 0x54 (hexadecimal)
Table 40. PWM MIN REGISTERS
Figure 52. PWM MAX Determines Maximum PWM Duty
Cycle Below the THERM Temperature Limit
Programming the PWM MAX Registers
The PWM MAX 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 MAX 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)
Register
0x64
0x65
0x66
Description
PWM1 Minimum Duty Cycle
PWM2 Minimum Duty Cycle
PWM3 Minimum Duty Cycle
Default
0x80 (50%)
0x80 (50%)
0x80 (50%)
Value = 128 (decimal) or 0x80 (hexadecimal)
Example 2
For a minimum PWM duty cycle of 75%,
Value (decimal) = 75%/0.39% = 85 (decimal)
Note on Fan Speed and PWM Duty Cycle
Value = 192 (decimal) or 0xC0 (hexadecimal)
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%
Table 41. PWM MAX REGISTERS
Register Description
0x38 PWM1 Maximum Duty Cycle
Default
0xFF (100%)
of its full speed. This is because fan speed in %RPM
generally relates to the square root of PWM duty cycle.
0x39
0x3A
PWM2 Maximum Duty Cycle
PWM3 Maximum Duty Cycle
0xFF (100%)
0xFF (100%)
Given a PWM square wave as the drive signal, fan speed in
RPM approximates to
% fanspeed + PWM Duty Cycle 10 (eq. 7)
Step 5: PWM MAX for PWM (Fan) Outputs
PWM MAX is the maximum duty cycle that each fan in the
system runs at under the automatic fan speed control loop.
For maximum system acoustic benefit, PWM MAX should be
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 )/100% PWM
intersection points.
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