Compact On-Chip Soft-Start Circuit for DC-DC Converters

An on-chip soft-start circuit consisting of a ramp-up voltage generator and an error amplifier with soft-start function is presented. The ramp-up voltage with slow ramp rate is achieved with small capacitors by using techniques of pulse swallowing and pulse current charging. The ramp-up voltage generator shuts down automatically to save power after the start-up has finished. The controlling scheme of start-up is realized by just adding two extra transistors to the conventional error amplifier circuit. In the start-up period, the output voltage of the converter and the current of the inductor rise are slowly controlled by the error amplifier output voltage which follows the ramp-up voltage. When the start-up finishes, a smooth transition from start-up to the steady states is achieved without disturbance to the current of the inductor, which always happens in the conventional method by using transferring switch. The soft-start circuit is applied to a peak-current mode boost DC-DC converter implemented with CSMC 0.5 μm BCD process. Simulation results show that the inrush current during start-up is eliminated effectively, and the output voltage rises smoothly without overshoot. Moreover, a smooth transition to the steady states without disturbance is achieved. The soft-start circuit is compact and suitable for on-chip implementation.