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TPS56528 Duty-Cycle Limitation

SimonF
Expert 3370 points
Other Parts Discussed in Thread: TPS56528

Hi Team,

when I try to simulate TPS56528 in Webench with 5V -> 3.3V @3A it tells me "Calculated duty cycle exceeds limit". In the TPS56528 datasheet I could not find any duty cycle limitations. Is this a bug in webench or am I missing something?

Best Regards,

SF

  • 0
    TI__Prodigy 575 points
    Hi, SF,

    I also tried to generate 5Vin -> 3.3Vo using WEBENCH, but i didn't get results, though i don't see 'Calculated duty cycle exceeds limit' ,
    5Vin, 3.3Vo is a general test condition.

    And from another perspective, datasheet claims minimum off time, 260nsec, from which you can calculated maximum duty cycle, around 80&, so it's fine for 5Vin, 3.3Vo.

    I think the problem for the WEBENCH may be that they didn't capture 5Vin data in the first place.
  • 0 in reply to Nick Chen
    TI__Expert 3370 points

    Hi Nick,


    thanks for your help! I can simulate 5V -> 1V for example so I think it should be possible with Webench for 5V input. This is what I get with 3.3V:

    If this is a bug can we please escalate to Webench team?

    Best Regards,

    Simon

  • 0 in reply to SimonF
    TI__Prodigy 575 points
    HI, SF,

    I'll align with WEBENCH team about it and keep you posted.
  • 0 in reply to Nick Chen
    TI__Guru**** 191445 points
    I can tell you a little about the history with this.

    TPS56528 uses DCAP2 control. It does not have an oscillator to set frequency. There is a minimum off time that limits the maximum duty cycle. For TPS56528 it is nominally limited to 78.55%. As the input voltage is decreased towards the output voltage, the duty cycle increases towards this maximum hard limit. However in actuality it is a soft corner. As the duty cycle approaches this limit, the output regulation degrades slightly. So Webench has a hard limit, which I think is set to 65% to have some margin. For Vin = 5 V and Vout = 3.3 V. the nominal duty cycle is 66%. However losses in the IC will tend to increase the duty cycle so the Webench calculated duty cycle will exceed its limit. You can defeat this by raising the minimum input voltage slightly in Webench.