>>
I think the feedback sense is reversed from what it should be: as is,
this circuit will latch up with Q1 either on or off, and stay there.
<<

Woops!  Good catch, thanks David!  As I said, I hadn't built this yet.  At
one point there was an additional inversion in there as I was evaluating
different topologies.  You just saved me a big "Doh!" after getting boards
back.

>>
By deleting R2, connecting +5VREF to the (-) opamp input, and
connecting the junction of R5 and R6 to the (+) opamp input and R4,
the regulator works fine (at least in SPICE).  With 20V in and 10V out
at one amp, efficiency looks to be about 80%.  Output ripple should be
about 850 mVp-p at this load, with an oscillation frequency of 6
kilohertz.  With input voltage reduced to 13 volts, efficiency goes up
to nearly 90%.
<<

Thanks again.  This matches very well with what I expected, although I
arrived at the characteristics by calculating time constants, currents,
voltages, etc.  It gives me more confidence in this circuit seeing theory
and simulation agree.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu