mc33063 current limiting
in [Design]
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From: Hammy on 6 Jan 2010 06:51 On Wed, 06 Jan 2010 06:39:03 -0500, Hammy <spam(a)spam.com> wrote: How not to build a constant current driver with the NCP3063. http://dev.emcelettronica.com/cheap-high-current-led-driver#comment-1782
From: Phil Allison on 5 Jan 2010 21:59 "Michael Robinson" > > How does the current limiting work in the MC33063. The datasheet doesn't > explain the actual mechanism. > http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF > Does it discharge the timing cap when it detects overcurrent, which would > restart the oscillator, or does it let the oscillator keep running but at > a lower duty cycle? ** OnSemi do have "app notes" you know !! http://www.onsemi.com/pub_link/Collateral/AN920-D.PDF See page 3 of the pdf, just under figure 4. Seems the current sense circuit ( Ipk Sense) INCREASES the current going to Ct and hence makes the output stage on time less. So the second of your hunches applies. ...... Phil
From: Jamie on 6 Jan 2010 19:48
Michael Robinson wrote: > How does the current limiting work in the MC33063. The datasheet doesn't > explain the actual mechanism. > http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF > Does it discharge the timing cap when it detects overcurrent, which would > restart the oscillator, or does it let the oscillator keep running but at a > lower duty cycle? > > My guess it would have to be duty cycle.. Current limiting takes precedence with the value of Rsc, (6) and (7) obviously are some internal comparator that will gate the duty cycle shorter when needed. If this isn't enough to maintain V, the comp for V just simply attempts a max duty cycle, which will only be what the current limiting circuit allows. Briefly looking at it, that is what I get from it.. |