Abstract: An ultra low quiescent current output-capacitor-less CMOS LDO chip is designed in this paper. This Circuit is based on a current-mode transconductance amplifier (CTA) as the error amplifier, and the stability of the circuit under various load conditions is ensured by miller compensation technology. While an instantaneous current-boosting voltage buffer is inserted to improve the transient responses. The stability and transient responses of the LDO regulator are presented, and it has been implemented and fabricated in SMIC 0.18 μm CMOS process. The measured results shown that the output voltage can be stable at 1.8 V when its supply is from 1.9 V to 3.2 V, the dropout voltage is just 35 mV, and the quiescent current is only 1.4 μA.