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Original question:

UC3842: Slope Compensation

UC3842: Oscillator charging ramp equations

J
Expert 1035 points
Part Number: UC3842

I think the oscillator charging ramp equations is wrong the Ichg is time variant  is not a constant how do the datasheet can get the delta Vosc 
can someone writhe more detail this data is hard to read. 
I want to know how to calculated 

  • 0
    TI__Mastermind 26395 points

    Hi

    The oscillator equation in the UC3842 data sheet is correct. The oscillator is a time varying voltage according to a linear ramp resulting from a constant current charging a fixed capacitor, where the constant current is determined from RT and the fixed capacitor is CT.

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Expert 1035 points

    the voltage is time varying why the current is constant? I =V/R the V is changing the current is constant? Do I misunderstand? 

  • 0 in reply to J
    TI__Mastermind 26395 points

    You are correct that V is changing, the current is constant. Instead of thinking about V=IR, consider the rate of change of capacitor voltage with respect to time:

    I=C(dV/dt) ---> dV/dt=I/C or dV=(I/C)dt

    The current is constant, the capacitor is constant and the voltage is time varying. The CT capacitor is charged (positive RTCT slope) by the constant current through RT up to ~1.7V (plus some small offset of ~0.6V). The CT capacitor is then discharged (negative RTCT slope) through the RTCT pin and the charge cycle repeats. This is the basic principle demonstrating how a PWM oscillator works.

    Steve M

  • 0 in reply to Steven Mappus1
    Expert 1035 points

    I understand what's you mean but I still confused about this part , I know the T chg is the charge capacitor time without adding the pulse , I don't understand the blue line mean and how to get the blue line formula

  • 0 in reply to J
    TI__Mastermind 26395 points

    For the purpose of setting up the UC3842, it is not necessary to derive the oscillator formula. If you apply the simplified formula given in the data sheet, the oscillator frequency you can expect will be as shown below:

    I believe we have established that the oscillator equation in the data sheet is correct and your question is more academic. Thanks for your question and if you need additional support using the UC3842, please open a new thread. Good luck with your studies.

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Expert 1035 points

    OK, I got it . thanks