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ADT7473
% fanspeed + PWM duty cycle
More than one PWM output can be controlled from a
single temperature measurement channel. For example,
Remote 1 temperature can control PWM1 and PWM2
outputs. If two different fans are used on PWM1 and PWM2,
the fan characteristics can be set up differently. As a result,
Fan 1 driven by PWM1 can have a different PWM MIN value
than that of Fan 2 connected to PWM2. Figure 54 illustrates
this as PWM1 MIN (front fan) is turned on at a minimum duty
cycle of 20%, while PWM2 MIN (rear fan) turns on at a
minimum of 40% duty cycle. However, both fans turn on at
exactly the same temperature, defined by T MIN .
100%
PWM2
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%
of its full speed. This is because fan speed in %RPM
generally relates to the square root of PWM duty cycle.
Given a PWM square wave as the drive signal, fan speed in
RPM approximates to:
10 (eq. 5)
Step 5: PWM MAX for PWM (Fan) Outputs
PWM MAX is the maximum duty cycle at which each fan
in the system runs under the automatic fan speed control
loop. For maximum system acoustic benefit, PWM MAX
should be as low as possible, but should be capable of
maintaining the processor temperature limit at an acceptable
level. If the THERM temperature limit is exceeded, the fans
are still boosted to 100% for fail-safe cooling (see
PWM2 MIN
PWM1 MIN
PWM1
Figure 55).
There is a PWM MAX limit for each fan channel. The
default value of this register is 0xFF and thus has no effect
T MIN
0%
TEMPERATURE
Figure 54. 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
anywhere from 0% to 100%. This allows the minimum
PWM duty cycle to be set in steps of 0.39%.
unless it is programmed.
100%
PWM MAX
PWM MIN
0%
The value to be programmed into the PWM MIN register is
given by:
T MIN
TEMPERATURE
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 80 (hex)
Example 2
For a minimum PWM duty cycle of 33%
Value (decimal) = 33/0.39 = 85 (decimal)
Value = 85 (decimal) or 54 (hex)
Table 44. PWM MIN REGISTERS
Figure 55. 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
Register
0x64
0x65
0x66
Description
PWM1 Minimum Duty Cycle
PWM2 Minimum Duty Cycle
PWM3 Minimum Duty Cycle
Default
0x80 (50%)
0x80 (50%)
0x80 (50%)
For a maximum PWM duty cycle of 50%
Value (decimal) = 50/0.39 = 128 (decimal)
Value = 128 (decimal) or 80 (hex)
Example 2
Note on 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
For a minimum PWM duty cycle of 75%
Value (decimal) = 75/0.39 = 85 (decimal)
Value = 192 (decimal) or C0 (hex)
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