Patent ID: 7579784

Claim:
A control circuit for controlling a switching element in a switching power supply by generating a switching signal, comprising: a hysteresis comparator that compares a feedback voltage determined by an output voltage of the switching power supply with a comparison voltage that makes a transition between two values in a hysteresis loop and generates a first enable signal asserted when the feedback voltage is lower than the comparison voltage; a soft start circuit which generates a soft start voltage that gradually increases for a period until the feedback voltage determined by the output voltage of the switching power supply reaches a predetermined reference voltage; a comparator that, when the soft start voltage is lower than a predetermined maximum voltage, generates a pulse width modulation signal by comparing the soft start voltage with an oscillation signal from an oscillator, and, when the soft start voltage reaches the predetermined maximum voltage, generates a pulse width modulation signal by comparing the maximum voltage with the oscillation signal; and a driver circuit that drives the switching element according to the pulse width modulation signal while the first enable signal is asserted, and stops the switching of the switching element while the first enable signal is negated, wherein the soft start circuit increases the soft start voltage at a first limiting speed when the feedback voltage is lower than a predetermined threshold voltage, and increases the soft start voltage at a second limiting speed which is set higher than the first limiting speed when the feedback voltage is higher than the threshold voltage.