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UCC28220: Max Duty Cycle Sub 60%

Jacob Olstad
Intellectual 1270 points
Part Number: UCC28220

Hi,

The datasheet specs the minimum max duty cycle to be 60% however, it seems possible to calculate out a duty cycle somewhere above 50% and below 60%.  Will the part work with a max duty cycle of say 55%  @ 200KHz switching frequency.

If not whats the limiting factor?

-Jake

  • 0
    TI__Expert 3095 points
    Hi Jacob Olstad,
    If you look at Equation (2) in the datasheet you can see that an output duty cycle of 60% requires that the oscillator has a duty cycle of 20%.
    An output duty cycle of 55% requires that the oscillator duty cycle is just 10% and an output duty cycle of 50% requires an oscillator duty cycle of 0%.
    Clearly the last of these is impossible.
    The smaller the oscillator duty cycle the greater the required ratio of discharging/charging current.
    I guess taking into account internal tolerances, across parts and temperature, it is not possible to guarantee that the oscillator frequency will always be much less than 20%.
    I hope this helps.
    Joe Leisten
  • 0 in reply to Joe Leisten
    TI__Intellectual 1270 points

    Hi Joe,

    That was my thought as well.  The datasheet only shows ~2% variation on output duty cycle overt emp and part variation which makes me think that there may be something else here or the part can handle a wider range.

    Is there some saturation in the oscillator that worsens the accuracy below 20% duty cycle?

    -Jake