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LM5156: design questions

Aki Li
Genius 9365 points
Part Number: LM5156

Customer was trying to design an isolated flyback with LM5156.

There were several questions as below, could you please help address them?

1.  How to understand the input and output relationship for the below OTA? When FB is connected to GND, what is the behavior for the OTA output? High impedance?

2. With the design calculator excel tool, how to obtain the secondary side turns value?

  • 0
    TI__Mastermind 26070 points

    Hi Aki,

    Thanks for using the e2e forum.

    1. This is related to the softstart behavior.
    During softstart, the SS voltage is compared to the FB voltage.
    After Vss reaches the level of Vref, the softstart is finished.
    From this point onward, Vref is compared to the FB voltage and SS voltage becomes irrelevant until the next startup.

    2. The winding ratio comes from the chosen transformer.
    The calculator recommends values for inductance and turns-ratio of the transformer, so the part can be chosen based on these specs.
    This app note gives additional information on this:
    https://www.ti.com/lit/an/snva866a/snva866a.pdf

    Best regards,
    Niklas