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TPS56221: PSPICE sim issue during startup and ac accuracy issue

Ella KIM
Expert 3942 points
Part Number: TPS56221

Hi,

Can you help advise on two strange operations? I don't think it's a stability issue. Do you think it would appear in bench as well? How do I improve it?

(1) overshoot and dip during the startup

(2) VOUT voltage dc accuracy issue - you can see dc value is a bit swinging.

  • +1
    TI__Mastermind 19635 points

    1) the 3.3nF soft start capacitor gives a very fast start up and the output voltage overshoots.     

    Look at the COMP pin voltage during this time, the response of comp pin will affect the vout. 

    2)  The output voltage ripple is from the inductor ripple current and output capacitor.   

    When the high side fet is on the inductor current ramp ups charging the output capacitor and load and the when the high side fet is off,   the capacitor supplies the load.  

    If the output capacitor had higher esr, the ripple voltage would be more similar to the inductor current. 

  • 0 in reply to David_Daniels
    TI__Expert 3942 points

    Regarding your reply (2), please let me clarify my question. Besides the Vout ripple, Vout DC level is swinging. you can see it repeats >0.8V and <0.8V.

  • +1 in reply to Ella KIM
    TI__Mastermind 19635 points

    It is not clear from the waveforms and schematic.   the compensation network looks okay and the only item

    I see is the gain at fsw which  is 15db with the 33pF cap. 

    I believe the compensation network may have a high gain at the switching frequency that is causing an oscillation at the cross over frequency.

    I think changing the 33pF C16 to 100pF will attenuate the gain at fsw and reduce the lower frequency oscillation.