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OPA4344 / Uncontrolled operation

S.Satoshi
Guru 21675 points
Other Parts Discussed in Thread: OPA4344

Hi All,

Our customer has evaluated OPA4344 at feedback circuit of AC/DC converter as attached. But the output of OPA4344 is uncontrolled suddenly though we have not found any trigger of this behavior. On the other hand, this behavior is fixed by adding RC filter on VCC of OPA4344.

Can you give your advice for the reason why this behavior is happened and it’s improved by adding RC filter on VCC?

Your quick response will be appreciated.

Best Regards,

Sonoki / Japan disty

 

OPA4344 abnormal operation.pdf
  • 0
    TI__Genius 15410 points

    Hi Sonoki,

    Did you try adding only a decoupling capacitor instead of an RC filter? Not using decoupling capacitors can give unexpected results. Please see http://e2e.ti.com/blogs_/b/precisiondesignshub/archive/2013/08/13/the-decoupling-capacitor-is-it-really-necessary.aspx

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    Guru 21675 points

    Dear Tim-san,

    Thank you for introducing the article about importance of decoupling capacitor.

    Now I'd like to have your further comments for this circuit. The OUT and +IN are biased from VCC line, so I think it may negatively affect for operation if some spike noise is observed on VCC pin and there is only small decoupling capacitor. So what do you think about this?

     

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Genius 15410 points

    Hi Sonoki,

    Yes, it is possible to see any noise spikes on the output if they are present at +IN. To help minimize noise spikes on +IN, place a capacitor from +IN (pin 10) to ground. The decoupling capacitors are there to give the op amp a steady power supply, I would recommend using a 0.1uF in parallel with 100pF for decoupling capacitors.

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    Guru 21675 points

    Tim-san,

    Thank you for your comment. I'll ask our customer to try to add capacitor on +IN and VCC.

    Best Regards,

    Sonoki