A Pulse-Skipping Soft-Start Circuit for DC-DC Converters

The rapid charging of the off-chip capacitor during power-up of the DC-DC converter could result in a large inrush current in the inductor, which could cause damage to other circuits. To suppress this phenomenon, decreasing the charging current would make the circuit less stable, and increasing the capacitance would make it more costly. Portable device applications such as medical and consumer electronics placed higher demands on the stability and cost of the converter. Therefore, a pulse-skipping soft-start circuit was designed in this paper. The current source was controlled to charge the capacitor intermittently by pulse-skipping so that the reference signal of the error amplifier rose slowly when the converter was powered up. Thus, the soft-start time was extended and inrush current was avoided. The converter was fabricated in a $\mathbf{0.18}\ \boldsymbol{\mu} \mathbf{m}$ BCD process with an area of about $\mathbf{0.885}\ \mathbf{mm}^{\mathbf{2}}$, and the soft-start circuitry accounts for only 1%. The measurement results show that the soft-start time of the converter is about 2.5ms, the output voltage rises smoothly, and the inrush current and overshoot voltage are effectively suppressed.