A transient-enhanced low-quiescent current low-dropout regulator with buffer impedance attenuation

This paper presents a low-dropout regulator (LDO) for portable applications with an impedance-attenuated buffer for driving the pass device. Dynamically-biased shunt feedback is proposed in the buffer to lower its output resistance such that the pole at the gate of the pass device is pushed to high frequencies without dissipating large quiescent current. By employing the current-buffer compensation, only a single pole is realized within the regulation loop unity-gain bandwidth and over 65 degrees phase margin is achieved under the full range of the load current in the LDO. The LDO thus achieves stability without using any low-frequency zero. The maximum output-voltage variation can be minimized during load transients even if a small output capacitor is used. The LDO with the proposed impedance-attenuated buffer has been implemented in a 0.35-mu m twin-well CMOS process. The proposed LDO dissipates 20-mu A quiescent current at no-load condition and is able to deliver up to 200-mA load current. With a 1-mu F output capacitor, the maximum transient output-voltage variation is within 3% of the output voltage with load step changes of 200 mA/100 ns.