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LM5156-Q1: Variable output voltage and compensation network

Francesco Rizzotto
Prodigy 50 points
Part Number: LM5156-Q1
Other Parts Discussed in Thread: LM5155, TL431

Tool/software:

Hi,

I am currently designing an isolated flyback converter and have reviewed the SNVA866A application note on the LM5155.

I would like to explore using an isolated amplifier in place of a phototransistor for feedback.

Additionally, I need to adjust the converter’s output voltage and am considering the current injection method outlined in the AN SNVAA33.

Could you please clarify the behavior of the LM5155 when the FB pin is connected to GND?

Also, how can the output voltage be modified using the COMP pin?

Thank you for your support!

Best regards,

Francesco

  • 0
    TI__Genius 13055 points

    Hi Francesco,

    Thanks for reaching out.

    Francesco Rizzotto said:
    Could you please clarify the behavior of the LM5155 when the FB pin is connected to GND?

    With this, the internal OTA is "disabled". It is sourcing the maximum current to the COMP pin.

    Francesco Rizzotto said:
    Also, how can the output voltage be modified using the COMP pin?

    The value at the COMP pin sets the dutycylce. So by influencing it, you could also set the output voltage.

    The method you mentioned works when you are using an error amplifier before COMP. So in case of a standart isolated flyback (with optocoupler), you would have to apply this method to the TL431.

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 50 points

    Hi Moritz,

    Thank you for your quick response.

    I plan to implement isolation using an error amplifier that monitors the output voltage, connecting its output to the COMP pin for compensation.

    This approach will allow me to control the output voltage by injecting current into the voltage divider of the error amplifier.

    However, I’m uncertain about the compensation calculations for this setup.

    Could you please provide a detailed explanation of how to calculate Rcomp and Ccomp?

    I've also attached an image to clarify my request.

    I appreciate your assistance!

    Best regards,
    Francesco

  • 0 in reply to Francesco Rizzotto
    TI__Genius 13055 points

    Hi Francesco,

    The compensation values depend on the powerstage, the error amplifier you use, and some parameters of the IC.

    I recommend tu read through the following document: https://www.ti.com/video/5099090674001 

    You can download the whitepaper in the video description. If you have further questions, please feel free to ask.

    Best regards

    Moritz