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UCC256404: Abnormal burst operation

user6455458
Prodigy 140 points
Part Number: UCC256404

Tool/software:

Hi, Team

Something different from normal is happening with the burst operation of UCC256404.
After Burst ON, it may turn Burst ON without turning Burst OFF. (For details, please refer to the attached waveform.)
Do you know the cause of this phenomenon?
Also, is there a way to improve this phenomenon?
For reference, I have attached a Design Calculator.

8814.UCC25640x Design Calculator Rev4.0.xlsx

  • 0
    TI__Guru* 86216 points

    Hello,

     

    I reviewed the data sheet and found this burst mode switching pattern figure below.  In your abnormal patent I believe the FBreplica voltage after the burst packet was complete transition to the BMTL threshold demanding another bust packet immediately.

     

     

    To verify this, you might want to study the comp pin along with this behavior to see how it relates to the burst packets.

     

    I am thinking there might be an issue with your loop compensation.  You might want to check this with a network analyzer to make sure the design has enough phase margin and gain margin.

     

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86216 points

    Hello,

     

    It was pointed out to me there is no comp pin on this device.   I reviewed the data sheet and it describes Vcomp as the output of the voltage compensator.  From the block diagrams this may be internal to the controller.

     

    The FB voltage does control the voltage level of FBreplica voltage.  So based on the voltage level FB you should be able to estimate when the device comes in an out of burst mode based on FBreplica voltage.

     

    Regards,