A Load-transient-Enhanced Output-Capacitor-Free Low-Dropout Regulator Based on an Ultra-Fast Push-Pull Amplifier

The paper proposed an ultra fast response output capacitor free (OCF) low dropout regulator (LDR) based on an ultra fast push-pull error amplifier. The amplifier detects the output voltage spike (the overshoot or undershoot voltage) of the LDR and generates an ultra-fast and large adaptive current to push up or pull down the gate of the power transistor to enhance the load transient performance. Based on quadratic I-V characteristics of MOS transistor, and adaptive biasing, the slew rate (SR) of amplifier in LDR is enhanced. The output driving capability is further improved by slew rate enhanced charging/discharging circuit. As a result, the amplifier performs transfer characteristics of much higher current capability and the output current is no longer limited by tail current or square-law in conventional designs. Designed in a 0.35 mu m CMOS process, the LDR is stable for 0pF to 50pF load capacitance and has 100mA maximum load capability with a quiescent current of 14 mu A. Simulation results including all process corners show that the undershoot/overshoot voltage are reduced to 147mV/57mV for load step from 1mA to 100mA in 100ns, and the recover time is only 210ns in worse case.