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UCC25600: I would like to know why the Recommend value of the Min value of R1 is 1 kohm.

Downey Kim
Expert 2675 points
Part Number: UCC25600

Hi TI team.

I would like to know why the Recommend value of the Min value of R1 is 1 kohm.

What is the concern when using RT resistance under 1kohm?

Thanks.

  • 0
    TI__Mastermind 40520 points

    Hello Downey,

    Thank you for your interest in the UCC25600 LLC controller.

    RT sets the switching frequency and lower values result in higher frequency.  However, the RT pin has an absolute maximum rating of 5mA (see section 6.1, page 4 of the datasheet).  I believe the recommended lower limit of 1K is intended to keep a comfortable margin from the Abs-Max current out of that pin.  

    Also, based on Eqn 2 (page 11) the fixed term of 150ns in the denominator means that reducing resistance results in non-linear diminishing increases in frequency, so it would require a larger drop in RT value to achieve a significant increase in frequency, and that pushes the current close to the Abs-Max rating.  

    Regards,

    Ulrich