U-168 Implementing a Practical Off-line Lithium-Ion Charger Using the UCC3809

The UCC3809 family of BCDMOS economy low-power integrated circuits contains all the control and drive circuitry required for off-line and isolated DC-to-DC fixed-frequency current-mode switching power supplies with minimal external parts count. The UCC3956 family of Switch-Mode Lithium-Ion Battery Charger Controllers accurately controls lithium-ion battery charging with a highly efficient average current control loop. This IC is designed to work as a standalone charger controller for a single-cell or multiple-cell battery pack. This application note describes a 20VAC off-line charger based on a two-series-cell Lithium-Ion pack with a 1200mAh capacity rating. The design described here can be modified to address different line and pack voltages. This application note discusses the recommended charge algorithm for the pack, primary and secondary circuitry design, feedback loop compensation, and magnetic design for the converter.