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UCC28220: UCC28220

Peter Almond
Prodigy 70 points
Part Number: UCC28220

I am looking to get a maximum output duty cycle of 90%, I have calculated the charge & discharge resistors based on a 300kHz oscillator frequency and get Rcharge = 53k & Rdischarge = 15k however in operation I don't seem to get as high a duty cycle as this, it seems to limit around 70%.  I have checked this isn't the current limit or any other part of the circuit.  Are the values I have calculated correct or have I made an eror somewhere.

Many thanks

  • 0
    TI__Mastermind 34465 points
    Hi Peter,
    To get a 90% duty cycle on the output you will require an 80% duty cycle on the oscillator.
    This is shown by equation 2 in section 8.3.3 of the data sheet.
    However also in this section it says that 80% is the maximum duty limit for the oscillator.
    I ran these equation for your application and I got the following values:
    Rdis = 13.6k
    Rchg = 54.4k
    I suggest you try these values and see how the Dmax behaves.
    Measure the pulse width directly on OUT1/OUT2 with the power supply running open loop and with LINEOV,LINEUV, CS1 and CS2 disabled.
    The equations for RCHG and RDISCHG may be so accurate at this upper limit of Dmax and you may need to adjust the resistor values to get the 90% output.
    Hope this helps
    Regards
    John