A VCO-Based Modulation Low Ripple PFM-PWM Buck Converter With Seamless Mode Transition

A voltage-controlled-oscillator(VCO)-based modulation PFM-PWM Buck converter is proposed to effectively reduce the output voltage ripple in PFM mode. The converter employs a VCO circuit instead of a hysteresis comparator to achieve PFM operation, ensuring a single inductor current pulse per cycle regardless of load current. A loop model for PFM mode is established for design of stability. Additionally, a hysteresis-based seamless mode transition method is incorporated to ensure converter consistent operating during full load range. Implemented using 0.18 \mu m BCD technology, experimental results demonstrate minimal output voltage ripple of 2.5mV in PWM mode and of 8mV in PFM mode with empty load at output voltage of 5V. Moreover, the converter achieves optimal efficiency of up to 92%.