A Compact Fully-Integrated Extremum-Selector-Based Soft-Start Circuit for Voltage Regulators in Bulk CMOS Technologies

A fully-integrated soft-start circuit for voltage regulators is presented in this brief. A soft-start strategy based on an extremum selector and a linearly ramped-up reference is developed to prevent any massive in-rush current through the power device during the start-up phase of the regulator. The strategy is realized by a compact on-chip circuit that requires no external components and has minimal transistor overhead, thereby minimizing the implementation area and the cost of the overall regulator. The linearly ramped-up reference is created through the use of nonlinear MOS capacitors available in a low-cost bulk CMOS technology. The proposed soft-start circuit has been implemented in a 0.35-μm bulk CMOS technology as part of a 50-mA linear regulator. The proposed circuit occupies 0.013 mm² on silicon, which corresponds to about 13% of the total area of the linear regulator. The soft-start circuit enables the output voltage of the regulator to be linearly ramped up to the steady state under different load currents and charging currents.