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Original question:

TPS54362-Q1: Question about output ripple

TPS54362-Q1: Question about output pulsated ripples

Farouk A
Prodigy 130 points
Part Number: TPS54362-Q1

I've got a similar problem with ripples ON OUTPUT OF TPS54362, my question is if the frequency FSW adjusted to be 555KHz, why the ripples frequency is 1.125MHz, REPEATED by 735Hz

how can I check my adjusted frequency? and what causes this problem?

PWR_) from 10V up to 31V, same output ripples

 

Circuit as follows

  • 0
    TI__Expert 6215 points

    Hello Farouk,

    The waveforms that you show indicate that the part is under very little load, a couple mA at most.  The 1 kHz frequency is likely your switching frequency.  The 1.25 MHz ripple is likely ringing in your PCB/ Oscilloscope combination.  Try probing directly across the output capacitor with your probe configured like on page 5 of the app note whose like is below:

    It the 1.25 MHz is still large, please let me know.  

    http://www.ti.com/lit/an/slpa005/slpa005.pdf

     

    If I have answered your questions, please click "resolved", otherwise, please reply to this post.

  • 0 in reply to Robert Blattner
    Prodigy 130 points

    Yes the load is less than < 90mA -100 mA

    I did the probing mentioned on page 5 of the app note in the link you gave ,  the 1.25MHz disappeared but still the glitches with little frequency 370Hz appear as picture bellow

    1: how to remove this ripples 
    2: how to make it work well in very low current loads

  • 0 in reply to Farouk A
    TI__Expert 6215 points
    Hello Farouk,

    It looks like the "hysteretic mode" setting on your board are not optimal. I calculate that your peak currents are huge. In order to fix this, you should use different values for the threshold resistors. There is a good explanation in the datasheet in section 7.3.19. I would limit ripple to around 60 mV. Another possibility is that the filter caps are too large. Keep the time constants of the filters affecting hysteretic mode to less than 0.4 us. If the calculations turn out to be difficult, please let me know.
  • 0 in reply to Robert Blattner
    TI__Expert 6215 points
    Hello Farouk,

    Did reviewing the hysteretic mode resistors help?
  • 0 in reply to Robert Blattner
    Prodigy 130 points
    sweeping input ascending bellow 10volt input no hystric mode nor ripples above 10V happens,

    recalculated again and make the threshold narrower so it is resolved
    Thank you,
  • 0 in reply to Robert Blattner
    Prodigy 130 points
    Thank you,

    To check hysteresis mode , make voltage input sweeping ascending : found that bellow 10V input no hysterics mode nor ripples above 10V happens,

    Recalculated again two resistor of the three responsible of the threshold made it narrower so issue of ripples is resolved
    and output now is very stable all the range of input regarding loads