Bob Ammerman wrote:
> I am looking for step-up switcher with following characteristics:
>
> 1: Input voltage: 1 alkaline cell, the lower the starting voltage and
> sustain voltage the better.
>
> 2: Output voltage: 5V
>
> 3: Current: max of 500ma (yes, I know this will kill the input cell
> pretty quickly)
>
> 4: Efficiency: not critical, 80% nice
>
> 5: Through hole mounting
>
> 6: Reasonable cost: In 100's total cost of circuit, including
> switcher, inductor, cap(s), diode under $2.00 if possible
>
> 7: Soft failure would be nice: ie: if it can't produce a full 5V on
> output given the source impedence then generate what it can.

A long time ago I did a discrete circuit for running a string of LEDs from
two AA batteries.  This was really a boost switcher with the feedback coming
from a low side current sense.  It would be a voltage regulator if the
feedback came from the output voltage instead.

It is basically a free running oscillator with the feedback killing the
oscillation when the output gets above the regulation threshold.  The
accuracy isn't that tight since the reference is a B-E junction, but this
could be changed of course.

I sortof remember this circuit or versions of it with different values
working down to 900mV.  There is very little left in a alkaline battery when
it gets down to 900mV.  Still, you'd have a easier time if you can use two
batteries in series.  If it's a space issue, how about two AAA in series
instead of a single AA?

Anyway, I put the schematic of the 20mA LED driver circuit at
http://www.embedinc.com/pic/light2/light2.pdf.  Parts of this circuit are
overkill for a 20mA output, but when you want up to 500mA input the extra
gain stages and the like are probably a good idea.  You will need a
different switching element that can take the higher current.  Figure peak
currents will be in the 1 to 1.5A range.

I did make boards of this circuit a long time ago, and can probably find a
few left over ones if you want.  This was meant for others build easily and
is all thru hole.  I can send you some if I can find them.

I just looked around and found 2 blank boards, and 2 where someone had
installed the parts.  If I remember right, one of those never worked (the
guy barely knew which end of a soldering iron to hold), and the other I
think worked fine.  I'd be happy to send you the whole mess if you want to
experiment with them.


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