Patent ID: 8143873

Claim:
A step-up switching power supply circuit which controls a first switching element based on an output signal of an error amplifier for detecting a deviation between an output voltage and a reference value, and which obtains the output voltage, stepped up by superposing a voltage based on energy accumulated in a coil on an input voltage, via an output terminal, the step-up switching power supply circuit comprising: a second switching element connected between an output side of the coil and the output terminal and comprising a P-MOSFET; a first diode having an anode connected to the output side of the coil; a second diode having an anode connected to the output terminal and having a cathode connected to a cathode of the first diode, a point of connection between the cathodes being connected to a back gate of the second switching element; a third switching element connected between the output side of the coil and the back gate of the second switching element and comprising a P-MOSFET, a back gate of the third switching element being connected to the back gate of the second switching element; a fourth switching element connected between the output terminal and the back gate of the second switching element and comprising a P-MOSFET, a back gate of the fourth switching element being connected to the back gate of the second switching element; delay means for delaying start of control for a stepping-up action by a certain time; constant-current drive means for driving the second switching element at a constant current within a delay period by the delay means; first switching control means for exercising on-off control of the first switching element and the second switching element alternately after a lapse of the delay period; and second switching control means for effecting switching control such that during halt of the control for the stepping-up action, or within the delay period, when the input voltage is higher than the output voltage upon comparison by a comparator, the third switching element is turned on, and the fourth switching element is turned off, or when the input voltage is lower than the output voltage upon comparison by the comparator, the third switching element is turned off, and the fourth switching element is turned on, and after the lapse of the delay period, the third switching element is turned off, and the fourth switching element is turned on, no matter which of the input voltage and the output voltage is higher when compared by the comparator.