LM2940 low dropout voltage regulator specs?
I will assume you are referring to this datasheet.
Under Section 6.5 Electrical Characteristics, the minimum input voltage is specified for 5mA &lt;= Iout &lt;= 1A over the recommended operating temperature range. You must give it 6.25V minimum for it to be able to provide that range of current over the temperature range.
The comparable number for 100mA is 5.35V (200mV max dropout + 5V + 150mV) over the temperature range . It would be more for 200mA. You don't really want to go right down to the minimum dropout, which is why it's specified with a bit more than the absolute minimum input voltage.
This is generally true of all linear ICs- if you run it right up against the maximum limits (for example, a rail-to-rail amplifier right near the supply rails) the performance will deteriorate noticeably.