reference
comparator ----
| |------------------------
anyways.
OK, it works in LTspice, with a generic opamp (whatever that may be :-) ):
Here is with 2 volt sine ripple superimposed on 12V battery, see how it finds the optimum setting: ftp://panteltje.com/pub/two_volt_ripple.gif The red line is the battery + ripple. The green line is the output voltage The cyan line is the output of opamp U2. That will go chaotic, just fine, ripple noise is chaotic too ;-) The chaos is filtered out in the loop filter.
Here is with half a volt ripple: ftp://panteltje.com/pub/half_volt_ripple.gif
And here with almost none (50 mV) ripple: ftp://panteltje.com/pub/50_millivolt_ripple.gif
See how close we get with the low dropout to the 12V battery voltage in this case! As I did set a maximum ripple with R15 / R16 (presented to comparator U2), the loop now longer is active, so 50 mV is allowed, and the MOSFET is 100% on. Or almost 100% anyways. Dunno if I will ever build this, more a proof of concept. Minimum dissipation in a linear series regulator.
Note that I disabled the short circuit protection by shorting the collector of Q1 against it's emitter. Measuring the drop over the MOSFET will not work here, those components should be removed.