> 1Khz etc was from the data sheet of the inductor, we are running with
> a 220pf cap which on a CRO gives 75khz frequency. circuit current is
> around 150ma.. would I be right in then saying..
> For 220 uH, 0.15A & 12-3.3 = 8v say.
> t = 220E-6 x 0.15/8 = 4.125 usec
>
> Does that sound better?

Yes.
75 kHz sets upper time limit on inductor charge time.

(Vchg* efficiency) /(Vout) ~=  Tout/Tchg

(8 * 0.6)/3.3 ~=  1.%:1 or 3:2
At 75 kHz ttotal = 13 uS so Tchg ~~~= 5 uS
Delta I ~= t.V/L = 5E-6 * 8 /220E-6 =~ 200 mA
= OK
Notice all the "~" s :-)

In continuous mode the inductor current is never zero and oscillates around
a mean level which is the average current.
You can expect peak inductor current to be say twice the eman (without
actually working it out).

> The catch diode is 10BQ030
> Vf = 0.3V

Specs look OK. 30V at 1A Schottky.

Efficiency is lower than I'd hope for.
As noted before, if possible try another physically larger inductor of about
the same inductance value "just to see". This would not be your final
component but would show you whether it had a major effect.

FLASHBACK -
You said
> I have built another version using a LM2674, and on vero
board it performs similar to the MC33063,

I assumed you meant that the efficiency was similar. If so, then there's
still something strange. If efficiency is higher then maybe all is OK.


    Russell McMahon

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