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UCC28070: CTA value choice

Hiroshi Doi35
Expert 2845 points
Part Number: UCC28070

Hi all,

Specsheet Rev.F Page 36 shows

CTA = CTB = 1/(RTA x fs x 3) --- (54)

Application note SLUA479B Page 9 shows

CTA = CTB = 1/(RSA x fs x 3) --- (33)

Which is better for CTA calculation?

  • 0
    TI__Mastermind 34465 points

    It look like the data sheet is correct and there is an error in the application note.
    Thank you for finding this discrepancy. I shall request that the application note be corrected

    Regards

    John Griffin

  • 0
    TI__Mastermind 25570 points

    Hello Doi-san

    RSA is the better value to use when calculating CTA.

    .

    Resistor ROA sets a DC offset (Voff) on CSA - The datasheet reccomends using 200mV as a starting value. ROA and RSA form a simple potential divider and once RSA (the current sensing resistor on the current transformer output) is set ROA may be easily calculated.

    RTA and CTA add a small ramp to the CSA signal. CTA charges at a rate given by the parallel combination of RTA and ROA and since RTA is much larger than RSA the parallel value is dominated by RSA. Typically RTA is 2.5kOhm, RSA is 32 Ohm and the parallel combination would be 31.6 Ohms.

    Thanks for pointing out the differences here.


    Please let me know if you have any further questions.

    Regards

    Colin