Abstract: Following the trend of integration in current-mode buck regulators,a soft-start circuit integrated on-chip is presented.A narrow pulse signal is generated by an oscillator.To get the ramp voltage,the on-chip capacitor is charged by a small constant current source with intervals.A multiplex comparator with low power dissipation is designed skillfully to limit the peak current,avoiding the inrush current and achieving soft-start.The presented circuit,which is concise and simple to implement,reduces the pin number and saves PCB space.A DC-DC buck converter with the proposed structure has been fabricated in a 0.5μm CMOS process for validation.The measured result shows that the chip starts up successfully in 140μs with 3.6V input,1.8V output,and 600mA load current.

Key words: soft-start circuit, buck regulator, current mode control, inrush current, integrated on-chip