This paper presents a novel A3COT buck converter with PICS technique to realize autonomous modulation for wider conversion rate and loading range. When the conversion rate changes from low to high or the current loading varies from light to heavy, the slope and amplitude of ramp-wave will be automatically modulated for stable output voltage. In the meantime, the turned-on time of high side MOSFET is also automatically modulated for the optimized efficiency and output ripple based on the conversion rate and current loading.