App note: Lithium Ion/Polymer portable power supply

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.

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  1. In the app note they are using a buck converter for 3.3V, not a linear regulator. Although you can use a linear regulator there, if current consumption is too high, it will be a waste of precious battery power.

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