Design Of A Cap-free Low-dropout Regulator

Integration of various functional modules lead the trend of system-on-chip (SOC)integration results in that more and more attention has been paid to the highperformance and low power dissipation power management system. A Low-Dropoutregulator (LDO) with low noise, low power dissipation so that is widely used inon-chip power management system. However, the LDO with large off-chip capacitormay not be favorable for on-chip applications that embed many voltage regulators.This makes the use of fully integrated output-capacitorless LDO (cap-free LDO)regulators in the power management system more popular. With cap-free LDOregulators, the resistive and inductive parasitic effects due to the external connectionand the huge number of I/O pads on the chip can be eliminated. Furthermore, theprinted-circuit-board layout area can be minimized. This raises the interest indeveloping the cap-free LDO regulators.In this paper, a cap-free low-dropout (LDO) regulator with improved transientSlew-Rate Enhancement circuit is proposed. The static error and transient error ofLDO can be controlled by using parallel structure constituted by and error amplifier.The loop error amplifier controls the static error and the loop constituted bySlew-Rate Enhancement circuit improves the system’s transient responses. TheSlew-Rate Enhancement circuit can boost the current for charging or discharging thecapacitor (Cpass) formed by the pass MOSFET immediately during the load transientand enhances the SR of pass MOSFET. These improve the transient response due tothe inhibition of the output voltage overshoot and undershoot.A cap-free low-dropout regulator based on0.35μm CMOS process is proposed.The experiment results show that the output of LDO regulator is1.78V,it can deliver50-mA load current and91mV dropout, the overshoot and undershoot are60mV and130mVrespectively when the load current ILchanges50mA transiently within0.6us,itcan recover to stable state within3us. It only consumes31.6μA, the line regulationand the load regulation are7.6mV/V and0.19mV/mA, respectively.