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. However, this scheme suffers from sensing circuit mismatch-induced instability, hence a concise passive ramp is proposed to mitigate the problem. The small signal model of the scheme is established to demonstrate the design standard of the system parameters. 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 μm CMOS process with a 4-MHz high switching frequency, this circuit occupies a total area of 1189 μm by 1199 μm in width and length including the pad ring. Finally, the experiment results show a fast load transient response, which is 42 mV voltage drop and 1 μs response time under the conditions of 1.1 V output voltage and 1 A load step-up with 1 A/500 ns slew rate. In addition, the frequency variation is 2% for a 1 A load step and there is no instability issue in this work.