Electron wrote:
> but when you read "24V input voltage, 50mA
> output current" and you don't have all the time in the world
> to read all data sheets with the due attention,

This is false logic, since you now apparently have time to investigate
alternate approaches.  Surely it would have been less time overall to do a
proper job up front and read the datasheet.  You did something irresponsible
and got caught.  Don't compound the error by trying to excuse it.  There is
no excuse.

As for your problem, converting roughly 27V or even 18V to a regulated 5V
sure sounds like a job for a switcher.  No, they don't need to take several
mA quiescient current.

One trick I've been using a bit lately is a switching preregulator for a
MCP1700.  Those are excellent little linear regulators as long as you can
live with no more than 6V in.  Note that they are some of the very few that
are stable with 0 ESR output caps (another little nugget you'd need to read
the datasheet for).

With 6V max in, 5V out, and some dropout voltage, there isn't a lot of room
for the input voltage.  The neat trick I've been using is to put a PNP
transistor accross the regulator input to output such that it turns on when
the input gets about 600mV about the output, which is right in the middle of
the optimum range.  The collector signal from this PNP is used to
enable/disable the pulse generator that drives the switcher.

It is possible to make a micropower oscillator with a few transistors, caps,
and high valued resistors.  Have that drive the switching element, then the
output of the voltage detect transistor accross the LDO kill the oscillator
when the input voltage is high enough.

Note that any quiescient current at the 5V end now shows up 3-4x lower at
the high voltage end.  With 27V, you need a switcher efficiency of 83% to
get 4x lower input current than the 5.6V current.  That's at the upper end
of easily doable with cheap parts.  3x current gain only requires 62%
efficiency, which should be quite doable.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist