An Ultralow-Power Fast-Transient Capacitor-Free Low-Dropout Regulator With Assistant Push–Pull Output Stage

An output capacitor-free low-dropout regulator (LDO) using a class-AB operational amplifier and an assistant push–pull output stage (APPOS) circuit to enable fast-transient response with ultralow-power dissipation is presented in this brief. The APPOS circuit is proposed to deliver an extra current that is directly proportional to the output current of the class-AB operational amplifier during transient state with an automatic on/off feature. Moreover, the small-signal and large-signal responses of LDO can be separately optimized. As a result, transient performances of LDO are improved significantly without requiring an area-consuming on-chip capacitor anymore. The proposed LDO has been implemented in a standard 0.35- $\mu\hbox{m}$ CMOS process. Experimental results show that the LDO can regulate the output voltage at 1.0 V from a 1.2-V supply voltage for the maximum load current of 100 mA. The output voltage fully recovers within 2.7 $\mu\hbox{s}$ with the load current switching from 100 $\mu\hbox{A}$ to 100 mA at a 1.2- $\mu\hbox{A}$ quiescent current.