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UCC38C43: Rt/Ct pin waeform question: should it be Linear or Exponential ?

Genius 3870 points

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Part Number: UCC38C43

Hi there,

we use UCC38C43 for simple flyback design. And I am confused about  RT/Ct pin waveform.

I think the charging and discharging wavefrom of Ct capacitor should be Exponential as said by datasheet section 8.3.5 "The exponentially shaped waveform charges up to a specific amplitude representing the oscillator’s upper threshold of 3V".  It is just a simple RC charging model.

However, figures 23 and 24 show us Linear waveform. Also, the detailed calculation on Unitrode app. notes  U-111 (TI slua110) page 6-10 all use the Linear graph.

Is this correct? or just an approximation for convenient ? Thanks.

  • Hi Yi Xiao,

    The real CT voltage graph is not linear, here is just for showing how how it works.

    Regards,

    Teng

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