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UCC25660: Burst mode thresholds

Jocelyn Harvey
Prodigy 20 points
Part Number: UCC25660

Hi, I'm trying to understand the various (VLLA-VLLB) options.. Why would someone want the HFBurstEntry threshold to be lower than the LFBurstEntry threshold? It seems it will never go into HF burst mode with (VLLA-VLLB) less than 1.087V. I just don't get it. Can someone explain?

Thanks!

Jocelyn

  • 0
    TI__Guru 70515 points

    Hello,

    Your inquiry is under review and I will get back to shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 70515 points

    Hello,

    This most likely has to do with audible noise.  When it comes to HFBurst this would occur at the higher power level.  As the load decreases to improve efficiency you would go into low frequency burst which is most likely in the audible range.  However, because the energy is lower there is less energy to produce audible noise.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 20 points

    Hello Mike, my question was more about the different programmable settings. It seems the HFBurstEntry threshold would be lower than the LFBurstEntry threshold with those settings.

  • 0 in reply to Jocelyn Harvey
    TI__Guru 70515 points

    Hello Jocelyn,

    As the load decreases from max load to no load the HF bust mode should be triggered first and low frequency should be triggered second.

    I found section 7.5.3.2 in the data sheet that describes the burst mode behavior.

    Regards,

    Mike

  • +1 in reply to Jocelyn Harvey
    TI__Intellectual 2425 points

    Jocelyn,

    You are correct. Under these settings, LFBurstEntry has higher threshold than HFBurstEntry. However, it doesn't necessarily mean that LF Burst will be triggered first. Actually, there is an internal logic to ensure the HF burst is always entered first, which is in the block shown below.

    Please refer to the following image as further illustration. As soon as HF burst is triggered, VFBReplica will jump first. In principle, you can have LFBurstEntry as high as where VFBReplica jumps show in the red circle. It is not encouraged to set LFBurstEntry higher than that point and a proper power stage design is needed. 

    I hope this clarifies and please let us know if there are further questions.

    Thanks