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Original question:

UCC28070-Q1: Resistor calculation

UCC28070-Q1: Resistor calculation

Mame TINE
Intellectual 305 points
Part Number: UCC28070-Q1
Other Parts Discussed in Thread: UCC28070

Hello,

You can calculate the RIMO value based on a modification of Equation 17 in the datasheet

The factor Iin(pk)*(Rs/Nct) in this equation is actually the max output voltage of the CT so you can replace this with the peak output voltage of your OPAMP.
Normally you would set this at about 3.6V to get the max range of the CSA,CSB inputs

The value of RSYNTH can be calculated from Equation 12 by noting that Rs/Nct is actually the load resistance seen at the primary side of the CT.
So you can replace Rs/Nct by the value of the current sense resistor (Rs) used on the source of the boost MOSFET.

This is the response of your but i want to know if the unit of sense resistor Rs is ohm or mili-ohm?

  • 0
    TI__Expert 6080 points

    Hello Mame,

    Thank you for your question.

    I would like to point you to the application note of UCC28070 (section 6) which will talk in depth about  how the calculations are done and the units of each component.

    Also, the link to the application note: 

    I hope this helps.

    Sonal

  • 0 in reply to Sonal Singh
    Intellectual 305 points
    Hi,
    I don't use a current transfomer but a shunt resistor. one of your colleague gave me the following response to calculate IMO SYNTH resistor :

    You can calculate the RIMO value based on a modification of Equation 17 in the datasheet

    The factor Iin(pk)*(Rs/Nct) in this equation is actually the max output voltage of the CT so you can replace this with the peak output voltage of your OPAMP.
    Normally you would set this at about 3.6V to get the max range of the CSA,CSB inputs

    The value of RSYNTH can be calculated from Equation 12 by noting that Rs/Nct is actually the load resistance seen at the primary side of the CT.
    So you can replace Rs/Nct by the value of the current sense resistor (Rs) used on the source of the boost MOSFET.

    Currently my question is to know if the unit of shunt resistor to perform calculation is ohm or mili-ohm.