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LM5155: COMP behaviour when FB is grounded

Alexander Risberg
Prodigy 20 points
Part Number: LM5155

I feel the datasheet is unclear in situations where the FB pin is grounded, such as in isolated flyback designs. Some application notes claim the error amplifier is disabled when FB is tied to ground.

In my design (isolated flyback), I had a pullup on the COMP pin to 6.85V, along with an optocoupler. The regulator worked fine even when I removed the pullup resistor, which means the COMP pin is actually sourcing current to the optocoupler. I can not find any information in the datasheet regarding the source of this current, is it coming from the error amplifier or is the current coming from another source within the regulator?

Best regards

Alexander

  • 0
    TI__Genius 11215 points

    Hello, 

    Thanks for reaching out. In the datasheet in the ABS MAX table you can find this info on the COMP:

    This can also be seen from eq. 27 in this document: https://www.ti.com/lit/an/snva866/snva866.pdf . The reason for the pullup is that the optocoupler might pull too low the voltage on the COMP pin causing bad dynamic response under certain operating conditions in the flyback. Let us know if this answers your question or if further support is needed. 

    Kind regards,
    EM

  • 0 in reply to EM
    Prodigy 20 points

    Hello EM

    The value you pointed out seems to be a limit on the maximum current the COMP pin can sink when the voltage exceeds the clamp. What I'm wondering is why the COMP pin is also sourcing a current, I though the only current source for the optocoupler was the pull-up resistor.

    Also, can you clarify what happens to the error amplifier when the FB pin is tied to ground? Is it disabled? If not, that would explain the current sourced by the COMP pin.

  • 0 in reply to Alexander Risberg
    TI__Mastermind 22365 points

    Hi Alexander,

    The EA is a transconductance amplifier (OTA), the output of the OTA is a current proportional to the voltage error. It can sink or source current.

    When the FB pin is tied to ground, current out of the OTA should be Gm*(Vref - FB) = 2mA. However, actual COMP sourcing current only can reach 180uA instead of 2mA.

    Best Regards,

    Feng Ji