MC34063 buck is VERY simple.
PIC buck is simple if you are a PIC master with real time switching
experience. Which you may be.
MC34063 could have low voltage shutdown added trivially easily  - I do
it using a jellybean PNP  + 2 resistors.
I'd expect the MC34063 buck with external MOSFET (or bipolar).would be
very easy to get going
ON Semi have slightly upgraded versions with a somewhat similar part number=
..

Note that 34063 has high side current limit which you could use even
if using an external switch.
Buck converter minimum drop is when output current flows through on
switch plus inductor.
Say under 0.1V if you try hard and a few tenths with not much effort at all=
..

As even 3 x 3.5V LEDs in series operating on 12V at 20 mA have around
R =3D V/I =3D (12-10.5)/0.02 =3D 75 Ohms in series, dropping 1.5V, adding a
mutual 0.1V or few is not going to make much difference to their
operating point.


 Russell.
--=20
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