ADT7476AARQZ-R datasheet(42/69 Pages) ONSEMI

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ADT7476A

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Programming the PWM Maximum Duty Cycle

Registers

The PWM maximum duty cycle 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 maximum

duty cycle 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 80 (hex)

Example 2:

For a minimum PWM duty cycle of 75%,

Value (decimal) = 75/0.39 = 85 (decimal)

Value = 192 (decimal) or C0 (hex)

Table 46. PWM MAXIMUM DUTY CYCLE REGISTERS

Description

Default

0x38

PWM1 Maximum Duty Cycle

0xFF (100%)

0x39

PWM2 Maximum Duty Cycle

0xFF (100%)

0x3A

PWM3 Maximum Duty Cycle

0xFF (100%)

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)/PWM100%

intersection points.

Figure 57. TRANGE Parameter Affects Cooling Slope

TMIN

100%

TEMPERATURE

PWMMIN

TRANGE

The TRANGE is determined by the following procedure:

1. Determine the maximum operating temperature for

that channel (for example, 70

°C).

2. Determine experimentally the fan speed (PWM

duty cycle value) that does not exceed the

temperature at the worst-case operating points. For

example, 70

°C is reached when the fans are

running at 50% PWM duty cycle.

3. Determine the slope of the required control loop to

meet these requirements.

4. Using the ADT7476A evaluation software, you

can graphically program and visualize this

functionality.

As PWMMIN is changed, the automatic fan control slope

changes.

Figure 58. Adjusting PWMMIN Changes the Automatic

Fan Control Slope

TMIN

100%

°C

33%

50%

As TRANGE is changed, the slope changes. As TRANGE

gets smaller, the fans reach 100% speed with a smaller

temperature change.

Figure 59. Increasing TRANGE Changes the AFC Slope

TMIN

100%

°C

10%

°C

TMIN−HYST

Figure 60. Changing PWMMAX Does Not Change the

AFC Slope

TRANGE

100%

10%

TMIN−HYST

MAX

PWM

ADT7476AARQZ-R Datasheet(PDF) 42 Page - ON Semiconductor

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