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LMR33630: About COMP Curve

GARAM AN
Intellectual 1990 points
Part Number: LMR33630

Hello,

I'm looking for the PFM and COMP Curve on the application note of LMR33630, I do not understand what the graph is.

Could you please explain what is the meaning at each stages(PFM, Auto Foldback, Clock Control Heavy Load Foldback)?

Can you explain why the graph line is steady on the PFM stage, and it's going up on the other stages?

http://www.ti.com/lit/an/snva857/snva857.pdf

Thank you

  • 0
    TI__Mastermind 22301 points

    Garam,

    This device family is peak-current mode control, meaning that there has to be a minimum peak SW current for proper control.

    This graph shows the peak current increasing with load, but the peak current cannot decrease to zero and stops at Ipeak-min. That is why it is flat in PFM mode.

    At light loads this is an issue because of the required SW current and very small output current, the excess current will overcharge the output voltage.

    In PFM mode, the SW current is ramped up to the minimum required peak current, and then there is a long no-conduction off-time so that the overall average current is lower. This is illustrated in the top graph in figure 8, note the dashed IOUT lines in these figures.

    In auto-foldback mode there is a valley current command shown by the grey line, the SW current must fall to that line during the off-time, and that controlled off-time will override the programmed frequency. Lines G and H in figure 9 illustrate this. Line I shows that at some point the ripple current will be larger than the peak-valley difference, and the device will just enter normal PWM without a forced fold-back.

    Clock control is a normal PWM.

    Heavy load foldback will force a valley current as described before, except this time as a current limit feature. The decrease in switching frequency will increase overall output currrent.

    Please look over again the figures illustrating the inductor current, output current, and switching. 

    -Orlando