He would like to know the minimum input voltage for 5V output.
I did not find this spec in the datasheet.
In general, all references have a certain amount of dropout voltage that
depends on the amount of current that the device is sourcing. Usually this is
anywhere from about 300 mV to 2V or so, meaning the input voltage would have to
be at least 300 mV to 2V above the output voltage in order to run properly.
I don’t see a specification for dropout anywhere on this datasheet, either, so
we’d have to measure it. The easiest way to do this would be to attach a load
to the output to get the maximum output current (about 15 mA), then set the
input voltage to maybe 7V or so and decrease it slowly until you notice the
output going out of regulation.