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LM3481: How to eliminate the high frequency jitter during the ramp up

HAN LEE
Expert 1515 points
Part Number: LM3481

Hi Experts,

Could you give us some suggestions for how to eliminate the high frequency jitter during ramp up ,thanks.

  • 0
    TI__Guru 68495 points

    Hi Han,

    Thank you for posting.  However, what is this waveform? Vout at startup? Can you show us your schematic, and tell us about the operating conditions?  

    Thanks,

    Youhao Xi,  Applications Engineering

  • 0 in reply to Youhao Xi
    Expert 1515 points

    Hi Youhao,

    Please see the sch file as follows,Vin : 5V , Vout : 12V, No load,thanks., the design has capability for 10A load.

    After that , we are following the all parameters of EVM board except inductor(now 17u H) to see whether it has same behavior,yes it was.

    The Vin 5V drop was dropping during ramp up ,we have checked the  

     

  • 0 in reply to HAN LEE
    TI__Guru 68495 points

    Hi Han,

    The schematic basically looks okay.  However, the current sense of 2mOhm appears to be too small.  I also noticed the inductor current peaks up to 20A in the above waveform.  Since Vin is supplied by your input source, it seems the 20A exceeds the source's current limit and it collapses. Can you double check your input source current limit?

    Thanks,

    Youhao

       

  • 0 in reply to Youhao Xi
    Expert 1515 points

    Hi Youhao,

    We have solved this issue after we put a smaller output cap (2200uf to 470uf),but I was wondering  why it cannot place a large cap on output,thanks.

  • 0 in reply to HAN LEE
    TI__Guru 68495 points

    Hi Han,

    A very large output capacitor will defeat the internal soft-start function.  The soft-start basically achieves two things by limiting the duty cycle: reducing the inrush current and also the output voltage overshoot.  Your large capacitor is not fully charged up after the internal soft-start is completed, and a large current is still needed, with max duty, to charge the capacitor, and the input current can exceed your source and bring the voltage down.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Expert 1515 points

    Hi Youhao,

    Thanks,understood.

  • 0 in reply to HAN LEE
    TI__Guru 68495 points

    Hi Han,

    Wish you good luck in your project.  Let me close this thread here, and feel free to write a new post when you have further questions.


    Best Regards,

    Youhao