> That, or even less, should do fine.  The deciding factors will be how
> high a peak current the transistor can deliver (Ipeak increases as
> inductance decreases) and how fast can it turn off.  I haven't

More exactly Wl = L*I^2/2 and at constant frequency I increases with the
sqrt of the L decrease, after neglecting other factors.

The problem is to get small chokes that are rated the specced inductance
at the DC current that a buck regulator passes through them. This usually
rules out normal small inductors, unless the current is very low. A
saturated inductor core can have 1/10 or less of the nominal inductance
and cause all sorts of trouble.

I once quickly put a 2W buck regulator together using parts from the junk
box and I remember that I tried at least 5 inductors that looked suitable
until I found one that was well behaved, and I had to rewind it with more
wire to be happy. It was an 'open' choke and it radiated like mad. The
strange part is that some of these seem to be used in office equipment
(unshielded plastic case) by some manufacturers, and the units bear FCC
class A or B compliance stamps (real ones, I checked).

Peter

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