Abstract: A soft-start circuit based on DAC (digital-to-analog converter) control for DC-DC switching regulator is presented.Using the techniques of DAC control and soft-start voltage sense,the rush current and overshoot of the output voltage during start-up are eliminated effectively,allowing for a smooth output voltage ramp from zero to regulation.At the end of start-up,the biased current of the soft-start circuit is cut off completely. Consequently,it is power-efficient.The circuit is designed using CMOS devices without the need of any external components,so it is suitable for integration in DC-DC switching regulators.The proposed soft-start circuit has been applied to a Buck PWM controller successfully.The experimental results show that in the entire load range the output of the system can start up smoothly,the inductor current remains stable,the output voltage rises smoothly without overshot,and the soft-start time is about 1.2ms.

Key words: soft-start, DC-DC switching regulator, rush current, overshoot