> Need a good way to power a PIC ('877) and a LCD (no backlight) from a
single AA cell.
>
> There are about 18 billion different switched regulators out there and I
can't seem to figure out which is best for
> this application.
>
> I'm expecing to draw around 4 mA running, and need to last a long time
quescent.


To help refine the answer, please answer the following (in triplicate on
form 922(b) ...)
You can expect an "OK" answer without knowing all the following but all the
following will help to get it really right.

One off or many off?
How many off?

How cost sensitive?

How size/weight sensitive?

How important is efficiency of energy conversion?
(50% OK, 80%, 90%, 95%)

Why a single AA cell. Do you mean eg Alkaline (Le Clanche) as opposed to
Lithium?

What voltages do you want / need?
Is PIC run at lowest possible power (3v 32 kHz) or at ???
(4mA expectation gives an indication)

Is this run 24 hours/non stop or turned on and off as required?
What is duty cycle?

and more ... ?

______________

Say its 4v mean needed at 4 mA (Allows some extra for LCD etc)
4v x 4mA = 16 mW
Assume alkaline AA = 3000 mAH.
Voltage = 1.5v start down to say 1v endpoint (0,9v OK but not too much
energy left).
Assume mean voltage of say 1.2V (curve droops faster at top)
3000 mAH x 1.2v = 3600 mWH available.
3600/16 = 225 hours at 100%
Say 200 hours to be safe.

190 hours at 95%
180 hours at 90%
160 hours at 80%

About 7 to 8 days continuous 24hrs/day.

______________________

Depending on answers above I'd consider either a Roman type 2 transistor
circuit or the 2 transistor multivibrator cct I posted within the last week
(slightly more parts, more "designable")  OR one of the purpose built single
cell inverter IC's on the market or just possibly a capacitor pump (but
provably not).

MAX1678 is a good example of a typical single cell to 5.5v max inverter IC.
As its available ONLY in their micromax package (SMD 0.65mm pin spacing) it
may not suit your need but otherwise it appears an excellent choice (I've
never used one myself). Synchronous rectification so high efficiency and
lowest possible parts count (IC, coil, 2 x cap (1 each at input and
output)), tiny. Starts on 0.85 volt. 37 uA quiescent or about 2uA on logic
shutdown.

________

In comparison my multivibrator needs two transistors, 3 resistors, inductor,
diode, zener, 4 capacitors (in, out, 2 x timing) and gives lower efficiency.

____________

Then something like the "Joule thief"  (Google will find) may suit your need
with some regulation added.
.



        Russell McMahon

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