Werner Terreblanche wrote:

> I was planning to use a MAX860 to double the voltage and then just
> use a low dropout fixed 5V regulator like the LM2936-5 to regulate it
> down to 5V.  My circuit only draws about 20mA, so the efficiency of
> the MAX860 should be more than 90%.   The MAX860 also provides a
> shutdown function, which should make it possible for the micro to shut
> down its own supply.
I'm not familiar with the MAX860 in particular, but doublers and
inverters tend to generate switching noise.
>
> How do the rest of you guys out there that makes battery operated
> equipment ensure a solid 5V over the entire life of the batteries?
> Is this proposed method that I want to use an acceptable way of doing
> this, or is there a better way to skin the cat?
National's low drop-out regulators are a good choice.  They make a
variety of devices so look for one with low temperature drift and good
line regulation. To avoid voltage drift problems, look at making your
analog circuitry ratiometric - e.g.: bridge transducer, voltage divider
reference (buffered and filtered).
>
> Another question : What is the best way to switch the 5V supply to
> other parts of the circuit.   I suppose it would be a MOSFET, but
> what I really want to know is the part number of a good general
> purpose MOSFET that is widely available.
National also makes a 5 pin regulator (LP2957AIT) that can have its
output switched on and off.  I'm just evaluating these now.  What has
worked for us in the past is a VFET (voltage switched FET) driving the
base of a PNP medium power transistor (the PNP acting like a switch).
If you over-saturate the transitor, the emitter-collector drop can be as
low as 300mV.  This is a constant voltage drop, whereas a FET is more
resitive in nature - your drop will be load-dependant.

--Matt