A primary-side control integrated circuit for AC-DC flyback converter with output voltage drop compensation is proposed in this paper. For mobile charging applications, the input voltage of mobile devices drops due to the equivalent resistance of universal serial bus (USB) cable. It will lower the charging performance and prolong the charging time. By the proposed control, the flyback converter can be operated in both discontinuous conduction mode (DCM) and continuous conduction mode (CCM), and the output current in each mode will be estimated by simple circuits. The voltage drop on USB cable can be compensated according to the output current. Therefore, the input voltage of mobile devices can be regulated under various load conditions to enhance the charging performance. Furthermore, primary-side control is adopted to reduce the circuit size and cost.