This document proposes a new output current estimation circuit for primary-side controlled LED drivers to improve line regulation. The circuit estimates output current for constant on-time control and can be used with existing flyback topologies. A prototype was built using a 0.6-μm CMOS process and tested at 100 kHz switching frequency with universal input and single-stage power factor correction. Experimental results showed the new scheme improved line regulation to within 1.5% and achieved up to 89.7% power efficiency.