ADT7462ACPZ-R7 datasheet(36/81 Pages) ONSEMI

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ADT7462

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Programming the PWMMIN Registers

The PWMMIN 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 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)

PWMMIN 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

Step 4—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 one PWMMAX limit (Register 0x2C) for all fan

channels.

Figure 56. PWMMAX Determines Maximum PWM Duty

Cycle Below the THERM Temperature Limit

TEMPERATURE

TMIN

100%

PWMMIN

PWMMAX

Programming the PWMMAX Register

The PWMMAX register (0x2C) is an 8−bit register that

allows the maximum PWM duty cycle for the outputs 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 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 maximum PWM duty cycle of 75%,

Value (decimal) = 75/0.39 = 192 (decimal)

Value = 192 (decimal) or 0xC0 (hexadecimal)

PWMMAX Register

0xC0 (75% default)

See the Note on Fan Speed and PWM Duty Cycle section

for more information.

Step 5—TRANGE for Temperature Channels

TRANGE is the range of temperatures over which

automatic fan control occurs when the programmed TMIN

temperature is exceeded. TRANGE is a temperature slope, not

an arbitrary value; that is, a TRANGE of 40°C holds true only

for PWMMIN = 33%. If PWMMIN is increased or decreased,

the effective TRANGE changes.

Figure 57. TRANGE Parameter Affects Cooling Slope

TEMPERATURE

TMIN

100%

PWMMIN

TRANGE

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