Patent ID: 8111051

Claim:
A control circuit of a synchronous rectification type of step-down switching regulator comprising a switching transistor and a synchronous rectification transistor, the control circuit comprising: an output monitoring comparator configured to compares output voltage of the switching regulator and a reference voltage that is a target value thereof, and to output an ON signal when the output voltage goes lower than the reference voltage; a light load mode detector configured to compare a switching voltage occurring at a connection point of the switching transistor and the synchronous rectification transistor, with a predetermined threshold voltage, and at timing at which the ON signal is outputted from the output monitoring comparator, to generate a light load detection signal having a first level when the switching voltage is a negative voltage, and to generate the light load detection signal having a second level when the switching voltage is a positive voltage; a pulse modulator configured to generates a pulse signal based on the ON signal and the light load detection signal so that (a) when the light load detection signal has the first level, the pulse signal has a predetermined level in a predetermined ON time from when the ON signal was outputted from the output monitoring comparator, (b) when the light load detection signal has the second level, the pulse signal is kept in a complementary level that is different from the predetermined level even if the ON signal was outputted from the output monitoring comparator; a driver circuit configured to generates a first and a second control signal based on the pulse signal and the ON signal, and to alternately turns ON, after a dead time, a switching transistor by the first control signal and a synchronous rectification transistor by the second control signal so that (c) the switching transistor turns on when the pulse signal transits to the predetermined level and the switching transistor turns off when the pulse signal transits to the complementary level and (d) the synchronous rectification transistor turns off when the ON signal is outputted and the synchronous rectification transistor turns on when the pulse signal transits to the complementary level.