• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 151 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS548A29: Switching frequency vs min on time

Michael Balourdas
Expert 4745 points
Part Number: TPS548A29

Hello team,

I know DCAP 3 control adjusts the switching frequency when the minimum on time restriction is violated to keep the output voltage regulated. My question is: is this recommended? Or should operating close to the minimum on time be avoided?

Specifically, my customer wants to run a 13Vin/.75Vout rail set at 800kHz. My math shows that the max fsw for these conditions is .75/13*1/(85ns) = 680kHz. Is there any issue with setting the switching frequency to 800kHz and having it run actually at ~680kHz while it is limited by the min on time? 

It looks like the datasheet has efficiency curves for 800kHz set point with 12Vin/0.6Vout which violates the min on time req so that's why I'm thinking it's okay, but would love to know your thoughts.

Thanks,

Michael

  • 0
    TI__Genius 16640 points

    The 85ns is the max  min on time, the typical  min on time is ~70ns. 

    On DCAP3, an on time boundary does not skip pulses like a fixed frequency control, but will extend the off time to stay in regulation.   

    This action will cause the switching frequency to be lower than expected.   

    The table below is the switching frequency with vin=12V  vs output voltage at the different setting

    Iout=3A 600kHz 800kHz 1000kHz
    Vout
    0.6 510 612 715
    0.9 550 698 796
    1.2 573 737 898
    1.8 619 804 993
    2.5 626 827 1025
    3.3 646 844 1050
    5.0 550 880 1098

    I think is okay to design with the 800kHz setting, but care needs to be applied when designing  the output filter realizing the ripple current will be greater.

  • 0 in reply to David_Daniels
    TI__Expert 4745 points

    Hi David,

    Really appreciate the quick response, thanks for the confirmation. We will be sure to design the output filter with the lower frequency in mind.

    Regards,

    Michael