Portable devices need an uninterrupted power supply both when the circuit runs on a battery, and when it is connected to a charger. This app note shows how to design a portable power supply outputting 5volts and 3.3volts.
The MAX1811 battery management IC handles the charging of a lithium ion/polymer battery. A DC/DC boost circuit provides a constant 5V supply from a battery voltage between 3.2V and 4.2V. The 5V supply is dropped down to 3.3V using a simple linear voltage regulator.
If only a single 3.3V supply is needed, the boost circuit could be replaced by a DC/DC step up/down converter.