This USB charger circuit is very simple and require very few external components . The charger schematic is based on the MAX1555 IC circuit manufactured by Maxim Semiconductor . This MAX1555 based charger can be used using the USB power or AC power  .
This charge is charging at 100mA from USB port and the maximum charging current increases to 350mA if the USB the AC adapter is connected  . If both power types are connected in the same time the AC adapter is automatically given precedence.
This USB charger ( based on the MAX1555 IC ) can be used for small devices like : PDAs ,Cell Phones , Digital Cameras , MP3 Players and other .

The charger can be used for Li-Ion or Li-Polymer batteries . The MAX1555 charger circuit require a input voltage between 3V and 7 V and deliver an 3.3 V output voltage .
As you can see in the schematic circuit , before the MAX1555 IC is connected the MAX8881 linear regulator IC which is capable to deliver up to 200mA .
The MAX1881 is specially designed for battery-powered applications where reverse battery protection and long battery life are critical.