A 4-MHz Ultra-Fast Transient Response Capacitor Current Adaptive On-Time (CCAOT) Controlled Buck Converter with Passive Ramp Compensation

This paper proposes a monolithic capacitor current mode adaptive on-time (CCAOT) controlled buck converter. A capacitor current-controlled modulation scheme ensures an ultra-fast load transient response but suffers from sensing circuit mismatch-induced instability. A concise passive ramp is employed to mitigate the problem. Moreover, a V-2-loop eliminates the circuit delay caused on-time pulse error and achieves a pseudo constant frequency. Implemented in a 0.18 mu m CMOS process with a 4-MHz high switching frequency, this circuit occupies an area of 1189 mu m * 1199 mu m with pad ring. The experiment results guarantee a fast load transient response during a 1A/500ns load transient. The converter can regulate the output voltage within 1 mu s with only 42 mV undershoot while the output capacitor is only 1.8 mu F. The switching frequency is varied only 2% with 1A loading current changed. Besides, there is no instability concern, unlike priors.