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UCC28701: is it ok to put a small capacitor between pin 5 & 6?

Kevin Lai
Intellectual 2916 points
Part Number: UCC28701

Hi,

there is a regulation problem and we found that placing a very small capacitor between pin 5 & 6 helps. in this design, the capacitance needed to fix the regulation is 27pF. is there any issue if we do this? 

datasheet warns a capacitor could interfere with accurate waveform sense. but any other risks? 

thanks,

Kevin 

  • 0
    TI__Guru* 85616 points

    Hello,

    That is the VS pin and it is recommended that  you use less than 3.3 pF on that pin.  If you are having a regulation issue chances are Aux winding is too noisy and or your layout may be susceptible to noise.

    Section 11 of the data sheet gives layout guidance and example of how to make a layout that is less noisy.

    It is critical that the Aux winding does not have noise on it to prevent a fault from trigger accidently.  The following link will bring you to an application note desribes the common issues with aux winding fault sensing and how to remove them.

    https://www.ti.com/lit/pdf/sluaac5

    Regards,

  • 0 in reply to Mike O'
    TI__Intellectual 2916 points

    Hi Mike!

    we did go over the layout originally with you and no issues were discovered. Also, done about 9 different transformer designs and the one currently used is the best. It seems to be working very well with the 27pF. What should the errant behaviour be if this capacitance is too high? What sort of tests could I do to prove or disprove it is a problem?

    Thanks,
    Kevin 

  • 0 in reply to Kevin Lai
    TI__Guru* 85616 points

    Hello,

    This could be transformer related if it is not layout.  

    Read the app note look at CS, Aux, Vout and Vdd.  If the aux wining is not clean you will a have regulation issues.  Having to put a 27 pF capacitor on the VS pin is an indication that the Vaux is noisy.

    Regards,

  • 0 in reply to Mike O'
    TI__Intellectual 2916 points

    Hi Mike,

    the layout seems fine. we did a review and there is nothing that stands out as the issue there. the transformer had 9 different iterations with all different winding arrangements, shielding, etc. and the best was selected.

    can you please ask the design team what the ramifications are to using 27pF? 

    Thanks,

    Kevin 

  • 0 in reply to Kevin Lai
    TI__Guru* 85616 points

    Hello,

    What can happen if the VS pin is heavily filtered is the following.

    1. The output  may not regulate correctly 

    2. Input under voltage may not work correctly because IVSL will go through the capacitor instead of coming out of the VS pin when the switch is on.

    If the designs layout is O.K. then there is noise being caused by the transformer.  You can see this and resolve this with snubbing and proper clamping as described in the following application note. https://www.ti.com/lit/pdf/sluaac5

    Regards,