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UCC28950: Is CS 1.18V increased due to added slope compensation?

Lauren Mayfield
Expert 7875 points
Part Number: UCC28950
Other Parts Discussed in Thread: UCC28951

The following is the test waveforms for

Yellow = Vct = Current Sense = CS

Blue = COMP

 

  1. The (COMP) - CS is 1.18V.  Is the 1.18V increase due to the added slope compensation?

 

  1. The ON time is 3.8us.   If 1.18V increase is due to the slope compensation, the compensation is 1.68V/3.8us = 0.3V/us.  Is it correct?

 

  1. Is the slope compensation equation and chart valid for all difference switching frequencies (see the copy below)?

  • 0
    TI__Expert 7875 points

    Any update on this? Thanks.

  • 0 in reply to Lauren Mayfield
    TI__Mastermind 25570 points

    Hi Lauren

    The slope compensation ramp will add to the CS signal as you noted but there is an 850mV offset applied to it which reduces the difference between CS and COMP from 1.18V to 330mV. Your slope compensation ramp  would then be about 330mV/4us (estimates from the images above) or about 82mV per us, implying a slope compensation resistor of about 55k (estimated from the graph)

    Figure 37 and the equation are valid for all switching frequencies - they give the slope of the compensation ramp, the amplitude of the ramp will depend on the switching frequency.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Expert 7875 points

    Hi Colin,

    Thanks for your response.

    Is the slope DC offset is positive or negative?

    For 850mV positive offset, when the current sense voltage Vss = 0.  The Vramp or Vcomp will be 850mV.  Is this correct?

    Thanks,

    Lauren

  • 0 in reply to Lauren Mayfield
    TI__Mastermind 25570 points

    Hi Lauren

    Q/ For 850mV positive offset, when the current sense voltage Vss = 0.  The Vramp or Vcomp will be 850mV.  Is this correct?

    A/ Almost correct. The PWM Comparator inverting input will be offset by +850mV with the slope compensation ramp added to it, the waveform will be a trapezoid. Vcomp is an independent variable with a level set by the error amplifier so that COMP will take up whatever voltage is necessary to keep the error to zero. Zero current would imply COMP = 850mV or less. In practice the UCC28950 will operate in Burst mode at low currents.

    BTW: I'd recommend that you use the UCC28951 device instead, it's a 100% compatible drop in replacement for the UCC28950 but it works better in high duty cycle, current limit conditions.

    Regards
    Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points

    Hi Lauren

    I think I've answered this query to your satisfaction - if you have further queries you can open a new post or open a linked post.

    Regards

    Colin