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UCC28C40: Light load behavior of UCC28C40

Evan Peng
Intellectual 790 points
Part Number: UCC28C40

Tool/software:

Hi expert,

This is Shenzhen FAE Evan, my customer found this PWM output below in light load scenario. Can you help clarify if the PWM wave is normal for UCC28C40 in light load scenario? I feel it seems go in pulse skip mode in light load, however I can not find any description about light load behavior of UCC28C40 in datasheet to prove it.

BR~

Evan

  • 0
    TI__Mastermind 24905 points

    Evan,

    What you are seeing might appear as pulse skipping but a controller that has true pulse skipping would do something analogous to switching at a minimum duty cycle and then skipping every nth pulse. Some controllers even change to hysteretic mode to achieve pulse skipping. There are different ways to accomplish this but it's done intentionally and in a controlled manner so that the output stays in regulation. Pulse skipping will also impact output voltage ripple but again, this is done in a controlled and predictable manner.

    When the UCC28C40 COMP voltage falls below the minimum threshold, pulses will stop until the COMP voltage returns. This is uncontrolled and somewhat random. Pulses are randomly missing as opposed to periodically skipping. The controller is doing what it needs top do to maintain light load regulation. To avoid missing pulses, you would need to add a minimum load...maybe start with scaling down the feedback resistors but some additional minimum load might still be needed.

    Regards,

    Steve