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Step 4—PWMMIN for Each PWM (Fan) Output

PWMMIN is the minimum PWM duty cycle at which each

fan in the system runs. It is also the start speed for each fan

under automatic fan control once the temperature rises

above TMIN. For maximum system acoustic benefit,

PWMMIN should be as low as possible. Depending on the

fan used, the PWMMIN setting is usually in the 20% to 33%

duty cycle range. This value can be found through fan

validation.

Figure 54. PWMMIN Determines Minimum PWM

Duty Cycle

TEMPERATURE

TMIN

100%

PWMMIN

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 PWMMIN value

than that of Fan 2 connected to PWM2. Figure 55 illustrates

this as PWM1MIN (front fan), which is turned on at a

minimum duty cycle of 20%, while PWM2MIN (rear fan)

turns on at a minimum of 40% duty cycle. Note: Both fans

turn on at exactly the same temperature, defined by TMIN.

Figure 55. Operating Two Different Fans from a

Single Temperature Channel

TEMPERATURE

TMIN

100%

PWM1MIN

PWM1

PWM2

PWM2MIN

Programming the PWM Minimum Duty Cycle Registers

The PWM minimum duty cycle 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%.

The value to be programmed into the PWMMIN register is

given by:

Value (decimal) = PWMMIN/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) l or 54 (hex)

PWM Minimum Duty Cycle Registers

(50% default)

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

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:

% fanspeed + PWM Duty Cycle

(eq. 5)

Step 5—PWMMAX for PWM (Fan) Outputs

PWMMAX 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, PWMMAX 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.

There is a PWMMAX limit for each fan channel. The

default value of this register is 0xFF and has no effect unless

it is programmed.

Figure 56. PWMMAX Determines Maximum PWM Duty

Cycle Below the THERM Temperature Limit

TEMPERATURE

TMIN

100%

PWMMIN

PWMMAX

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