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LM5156-Q1: How to select FB point on PCB board

Elec Cheng
Expert 7055 points
Part Number: LM5156-Q1

Hi, 

Two questions:

1.Is the output voltage ripple on A different from on B or not? If not, why? .

2. FB pin is used to feedback and stabilize output voltage. So which point, A or B should it connect to?

Thanks and Best Regards,

Elec Cheng

  • 0
    TI__Guru 63025 points

    Hello Elec,

    Thank you for considering our device.

    1. If you consider this as a connection in layout, the ripple at A is a lot higher than at B.

    2. B is the right point to connect. The diagram above is not meant to be layed out as is. Sorry if this caused confusion. It was just easier to realize having the feedback divider connected like this as in the pure schematic world, A and B are connected to the same node.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    TI__Expert 7055 points

    Hi Brigitte,

    thanks for your clarification. Two more questions:

    Commonly, output capacitor is made up of several paralleling capacitor. There will be several points from A to final output point. In this case, will the ripple gradually decreases along these points? And should the FB point be connected to the final output points still?

    Thanks and Best Regards,

    Elec Cheng

  • 0 in reply to Brigitte
    TI__Expert 7055 points

    One more thing, is FB connecting to point B recommended for most situation? Is there any case to connect A point? 

    Thanks again!

  • 0 in reply to Elec Cheng
    TI__Guru 63025 points

    Hello Elec,

    In general it is not recommend to connect the feedback node in any case to point A. Point A is very noisy.

    It is more difficult to answer your question about which connection on the various output capacitors is the best choice for the feedback node. If you have a very noisy load and you want to remove that noise from the feedback point, it is recommended to connect somewhere in the middle of the output capacitors.
    if you want to have the fastest reaction time on a load change, it is recommended to connect as close as possible to the load, so after the last capacitor.

    Best regards,
    Brigitte