ADT7467ARQZ-R7 datasheet(44/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 44 of 77 | www.onsemi.com

Programming the PWMMIN Registers

The PWMMIN 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 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 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)

PWMMIN Registers

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 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

cycle

duty

PWM

speed

fan

STEP 5: PWMMAX FOR PWM (FAN) OUTPUTS

PWMMAX 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, PWMMAX 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 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, therefore, has no effect unless

it is programmed.

TEMPERATURE

TMIN

100%

PWMMIN

PWMMAX

Figure 60. PWMMAX Determines Maximum PWM Duty Cycle Below the THERM

Temperature Limit

Programming the PWMMAX Registers

The PWMMAX 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 PWMMAX register is given by

Value (decimal) = PWMMAX/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)

PWMMAX Registers

(100% default)

See the Fan Speed and PWM Duty Cycle section.

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