ADT7490ARQZ-R7 datasheet(37/75 Pages) ONSEMI

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ADT7490

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Figure 51. Operating Two Different Fans from a

Single Temperature Channel

TEMPERATURE

TMIN

100%

PWM1MIN

PWM1

PWM2

PWM2MIN

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

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. 7)

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 52. PWMMAX Determines Maximum PWM Duty

Cycle Below the THERM Temperature Limit

TEMPERATURE

TMIN

100%

PWMMIN

PWMMAX

Programming the PWMMAX Registers

The PWMMAX 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 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% = 85 (decimal)

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

PWMMAX Registers

Step 6: TRANGE for Temperature Channels

TRANGE is the range of temperature over which automatic

fan control occurs once the programmed TMIN temperature

has been exceeded. TRANGE is the temperature range between

PWMMIN and 100% PWM where the fan speed changes

linearly. Otherwise stated, it is the line drawn between the

TMIN/PWMMIN and the (TMIN + TRANGE)/100% PWM

intersection points.

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