A soft start circuit for automobile voltage regulator

A soft-start circuit for automobile voltage regulator is presented in this paper. By using a DAC control and frequency detection technology, this circuit can provide a 17% pre-excitation duty ratio to start the automobile voltage regulator. When the alternator achieves the cut-in rotational speed, the soft start circuit increases the duty ratio of the driving pulse for the switching transistor from 17% to 100% gradually, which makes the magnetizing current of the alternator to increase gradually accordingly and suppress the burst of the mechanical torque, avoid engine stalling due to an instantaneous overload. Also it has Soft loading function. Based on the CSMC 0.5um 40V double poly BCD process, the soft start circuit is implemented on chip, the electrical characteristics of the design is verified in Spectre simulator in the Cadence environment. The time needed for the transition of the duty ratio from 17% to 100% is 4 seconds.