> Bob Blick will offer you his boost buck SEPIC circuit, and may even
> make helpful comments as well as long as you remember not to mention
> global warming (or climate change).

Here it is
SEPIC converter that in this example takes 8 to 16 volts in and 
produces 12V out. This can easily be adapted for your need.

        http://www.bobblick.com/techref/projects/mp3book/mp3book2/mp3book2.html

As an alternative to the gapped RM10 core you could use a powdered 
iron toroid such as the wide range of well priced products from 
micrometals. NB NOT a ferrite toroid.

For LED driving, rather than the voltage feedback that Bob uses here 
(10k and 2k7 divider) you could use current feedback by using a sense 
resistor in the ground return of the LEDs. As the circuit shown uses a 
feedback voltage of 27/(10+2k7) * 12 = 2.56V you need a buffer between 
the LED sense resistor and the feedback input as you don't want to be 
dropping 2.6V in the sense resistor.

So - sense resistor = about R = 0.6/I_LED
For I_LEd = 300 mA say you get 0.6/.3 = 2R.
For I_LED = 1A you get R = 0R6.

Drive an NPN jellybean transistor base with this and use the collector 
to simulate reference feedback.
More on a full circuit for this purpose available if you decide this 
may be useful.

Bob's UC3843 is fine for this task (especially as he's already done 
the work getting it going) but other ICs like the cheap and highly 
available MC38043 would also work well.

There's an 820 uF (!!!) 'coupling' capacitor in the SEPIC cct due to 
the 8A power output, but a lower value cap is going to be OK for lower 
powers. (As it's an electrolytic it's not a major issue anyway).

He uses the utterly lovely MBR1645 Schottky output diode. Digikey has 
these in stock You'll need to use something of equal niceness (or 
utter loveliness :-) ) as well but can use a lower current rating if 
desired. (Using a higher current rating than necessary has little 
disadvantage except component cost. Lower voltage Schottky's will have 
a lower forward voltage drop at the same current all else being 
equal). The Schottky diode is used for it's very low forward voltage 
drop and reasonable high frequency performance. You can use high speed 
switching diodes in this role but a higher forward voltage drop. This 
has about 0.6V drop at typical currents. You can halve that with very 
low voltage parts. This is 16A 45V rated, as the part number suggests. 
For other suitable Schottky's search Digikey (or your favourite 
seller) for parts with part number MBR.... . There are other Schottky 
families but that's an excellent start.


        Russell 

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