How close to drop out voltage can one operate an LDO in order to squeeze the
last ounce out of my circuits efficiency?
The minimum headroom required is highly dependent on the LDO characteristics,
load current, and the use case. For instance, if rejecting noise that appears
on the input to the LDO is important, the minimum headroom will probably be
several hundred to 500mV or even more. If on the other hand PSRR of the LDO is
not a consideration, then theoretically, the LDO may be operated with only a
few millivolts of headroom. In practice however, unless the operating current
is very low, at least 100mV to 200mV will likely be required to accommodate the
tolerances of the source voltage and the LDO output voltage.