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Roman wrote...
>Dave Dilatush wrote:
>
>> In addition to the LT1930 that Andy Kunz mentioned, Maxim also
>> has a number of chips that perform this function, such as the MAX
>> 608 and the MAX1522/3/4 series.  None are terribly expensive, and
>> all are easy to use and perform well.
>>
>> Some things just aren't worth "rolling your own" anymore, other
>> than for the sheer joy of dabbling.
>
>Unless of course you are a manufacturer and
>want to replace a $1.50 chip with $0.06 worth
>of transistors. ;o)

Well, sort of: as Russell pointed out, the circuit he posted
(smps512.gif) isn't complete yet since it has no means of
regulating its output voltage.  For one possible complete design
based on what Russell posted, see the attached.  In this case,
I've (more or less) optimized the circuit to generate a 5 volt
output from a single alkaline cell.  Zener diode D2 sets the
output voltage by providing feedback through transistor Q3.
Diode D3 has been added to prevent B-E reverse breakdown of Q2
during flyback.  I haven't built it but simulation suggests it
ought to work, with roughly 60% efficiency at 5 mA load current.


So we've got 3 transistors, 2 diodes, 1 Zener diode, four
resistors and 2 capacitors.  That's a dozen components, versus
whatever the IC solution takes.

This discrete solution is probably still cheaper than what the IC
and its related components would cost, but a manufacturer (at
least, any manufacturer who's smart enough to stay in business
very long) will have other considerations besides the purchase
price of the components when comparing alternative designs.

Component count and circuit board area may be an issue, as well
as reliability- the more parts a design has, the more likely
something will fail.  If one design has a lot more components
than another, will the extra components fit in the available
space?

Production volume is a consideration, too: is it worth spending
several weeks (maybe even more) of engineering time to come up
with a robust switching regulator design using discrete
components, when a single IC can do the job?  If volume is high
enough, the answer may be "Hell, yeah!"; if volume is low, it
might be "Hell, no!"; and in between, the question bears careful
examination.

Time to market may be another consideration.  Is it really
worthwhile tying up engineering manpower working on a circuit
function that could just as easily be purchased as a chip,
instead of attending to other parts of the design that may be
much more crucial to the product's success?  Sometimes the extra
delay in getting a product out on the market is acceptable; other
times it's not.

Circuit performance may be an issue- indeed, it may be the
biggest issue in something like this: most of the IC switching
regulators available today will outperform one of these
"3-transistor wonder" circuits by a wide, WIDE margin in terms of
line and load regulation, output voltage accuracy, temperature
stability, efficiency, input voltage range and fault tolerance.
For some applications, the 3-transistor wonder may be adequate;
for others, it may fall pathetically short of being good enough.

What I like to call "quality of design" comprises a set of issues
that affect production yield and field return rate- things like
design margins, component de-rating, sensitivity to component
value variations (both unit-to-unit and over temperature),
typical characteristics vs. worst-case, adherence to design rules
and the like.  One design rule this 3-transistor wonder circuit
runs afoul of (specifically with regard to Q1) is pretty
common-sense and widely observed: never rely on transistor beta
as a means of limiting current flow, because it varies too much
from unit to unit, from lot to lot, and over temperature.

Experience has taught me over the years that it frequently pays
to overdesign voltage regulators- that is, to intentionally make
them better than apparently required.  Operating voltage tends to
affect many aspects of circuit performance in subtle,
hard-to-troubleshoot ways and a good, tightly regulated supply
voltage in a product can make for a lot fewer production
headaches.  Just my observation; YMMV, of course.

DD

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