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ADT7467
More than one PWM output can be controlled from a
single temperature measurement channel. For example,
Remote 1 temperature can control PWM1 and PWM2
relates to the square root of the PWM duty cycle. Given a
PWM square wave as the drive signal, fan speed in RPM
approximates to
outputs. If two fans are used on PWM1 and PWM2, each
fan’s characteristics can be set up differently. As a result,
% fan speed + PWM duty cycle
10
Fan 1 driven by PWM1 can have a different PWM MIN value
than that of Fan 2 connected to PWM2. In Figure 58,
PWM1 MIN (front fan) is turned on at a minimum duty cycle
of 20%, and PWM2 MIN (rear fan) turns on at a minimum of
40% duty cycle; however, both fans turn on at the same
temperature, defined by T MIN .
STEP 5: PWM MAX for PWM (Fan) Outputs
PWM MAX is the maximum duty cycle that each fan in the
system runs at during the automatic fan speed control loop.
For maximum system acoustic benefit, PWM MAX should be
as low as possible but capable of keeping the processor
below its maximum temperature limit, even in a worst-case
scenario. If the THERM temperature limit is exceeded, the
100%
PWM2
fans are still boosted to 100% for fail-safe cooling.
There is a PWM MAX limit for each fan channel. The
default value of this register is 0xFF and, therefore, has no
effect unless it is programmed.
PWM2 MIN
PWM1 MIN
0%
T MIN
PWM1
TEMPERATURE
100%
PWM MAX
T MIN
Figure 58. Operating Two 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
from 0% to 100%. This allows the minimum PWM duty
cycle to be set in steps of 0.39%.
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)l or 0x54 (hexadecimal)
PWM MIN 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)
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 runs the fan at closer to 50% of its full
speed, because fan speed as a percentage of RPM generally
PWM MIN
0%
TEMPERATURE
Figure 59. 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
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)
Value = 128 (decimal) or 0x80 (hexadecimal)
Example 2: For a minimum PWM duty cycle of 75%,
Value (decimal) = 75%/0.39% = 192 (decimal)
Value = 192 (decimal) or 0xC0 (hexadecimal)
PWM MAX 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)
See the Fan Speed and PWM Duty Cycle section.
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