Patent ID: 7262582

Claim:
A switching power supply circuit comprising: a stepping-up DC-DC converter; a comparing portion; a driving portion: a constant voltage portion; a switching portion; an activating/deactivating portion; and a soft start circuit, wherein the comparing portion compares a reference voltage with a voltage based on an output current of the stepping-up DC-DC converter, wherein the driving portion performs switching control to turn on/off a switching device provided in the stepping-up DC-DC converter according to an output of the comparing portion, wherein the constant voltage portion feeds a constant voltage as a drive voltage at least to the comparing portion, wherein the switching portion switches constant voltage feeding operation of the constant voltage portion between an activated state and a deactivated state according to an external signal, wherein the activating/deactivating portion switches switching control operation of the driving portion between an activated state and a deactivated state according to the external signal, while preventing the switching control operation of the driving portion from being switched from the deactivated state to the activated state from a time point that the constant voltage feeding operation of the constant voltage portion is switched from the deactivated state to the activated state until a predetermined period elapses, and wherein the soft start circuit has a capacitance and a discharging portion, charges the capacitance when the switching control operation of the driving portion is started and thus an output voltage of the stepping-up DC-DC converter rises, controls the driving portion according to a voltage across the capacitance in such a way that, when the output voltage of the stepping-up DC-DC converter rises, the output voltage of the stepping-up DC-DC converter rises gently regardless of the output of the comparing portion, and makes the discharging portion discharge the capacitance when the switching control operation of the driving portion is in the deactivated state, completion of discharging of the capacitance requiring a period longer than a cycle with which the activating/deactivating portion switches the switching control operation of the driving portion between the activated state and the deactivated state.