>>> So lets say I leave it at the default 5 volts output. What happens to
>>> the voltage output if I decrease the input voltage to 4 volts, for
>>> example?
>>>
>>> Will the output be exactly 4 volts? Or does it work a different way?

> All in all it is much better to assume anything can happen when the input
> voltage drops below the minimum spec level.

Yes.
Unless the manufacturer has specified what happens.
Some regultaors have a nasty high input current range when Vin < 
Vout_min_operating.
Others track Vin quite closely, as has been noted, down to a certain limit 
then delta-V drops sharply below that. Others are more graceful.

A few days ago I came across this Ricoh LDO data sheet

    http://www.datasheetpro.com/30212_download_R1154H119B_datasheet.html

It does  afar far better job than many of describing operation under various 
mixes of paramaters and in out of regulation conditions (page 9).
With a datasheet like this you can have some confidence* that the device 
will behave OK below with below regulation Vin. Without this level of detail 
any assumptions are variably dangerous. YMMV.
* Caveat emptor.

This device seems quite good but not perfect withing its application range 
(eg it's dropout gets a bit non LDO at currents near its top end). But 
overall it could be a useful device. It doesn't seem to be readily available 
"in the West", but a call to your friendly Ricoh sales office may produce 
results. Or maybe not. .


           Russell


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