Is it reliable to use Li-ion battery + LDO for analog circuits and 3.3v digital circuits?
If you really want to discharge the battery to voltages below 3.5 V, you won't be able to use an LDO regulator, since it needs to be powered with voltage higher than it outputs. However, considering how little charge remains in the cell below 3.5V, I would consider using the LDO anyway and stop the discharge earlier.



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Granted, your system will run about 20% less time on battery, but it will be much safer, since a Li-ion cell discharged to 2.7V is very vulnerable: leaving it uncharged for a couple of weeks will drop the voltage even lower, resulting in permanent damage.