This paper presents a tri-mode buck converter with seamless mode transition. An auto-transition on-time generator is designed to realize automatic transition between PWM and PFM modes. In order to achieve low quiescent current and high efficiency under ultra-light load, an ECO mode is proposed. An adaptive main comparator is presented with its bias current and feedback resistors dynamically adjustable to realize load-dependent seamless mode transition between PWM/PFM and ECO modes. The proposed tri-mode buck converter is implemented in 180nm CMOS process with 1μA to 50mA (50000X) load range. The minimum quiescent current is 486nA and the maximum efficiency is 92%. Also, the efficiency remains over 85% from 0.1mA to 50mA.