Bob Blick wrote:
>> 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?
>
> Almost 4 volts.

Not so fast.  While this appears to be true for most of the LDOs I've seen,
it is not guaranteed.  The datasheets don't say what happens when the input
voltage drops below spec.  "Low dropout" only refers to the operating range.
4V into a 5V regulator is not in the operating range.  It may take a few
volts input for the output to come on at all.  Then there could be stability
issues because the transistors aren't biased as intended and impedences are
different.

That said, LDOs generally have a MOSFET pass transitor.  For voltages just a
little below the input spec, it is usually biased on.  But at some point
there isn't enough bias voltage to keep it on enough, and the circuit that
drives it won't be able to operate.

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


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