Control circuit and method for maintaining high efficiency in switching regulator

A control circuit for controlling a switching regulator circuit for supplying a regulated output voltage, the switching regulator comprising an inductor, a first switch coupled between an input voltage and a first terminal of the inductor, a second switch coupled between GROUND and the first terminal of the inductor, a third switch coupled between a second terminal of the inductor and GROUND, and a fourth switch coupled to the second terminal of the inductor and the regulated output voltage, the control circuit comprising:

• an error amplifier, comparing a feedback signal with a reference voltage to produce an output voltage, the feedback signal being proportional to the regulated output voltage;
• a waveform generator, for providing first and second periodic waveform signals;
• a first comparator, comparing the output voltage of the error amplifier with the first periodic waveform signal to generate a first control signal;
• a second comparator, comparing the output voltage of the error amplifier with the second periodic waveform signal to generate a second control signal;
• a clock generator, for generating a first clock signal;
• a frequency divider, for dividing frequency of the first clock signal to provide a second clock signal;
• and
• a control logic, in response to the first control signal, the second control signal, the first signal and the second clock signal, for providing first, second, third and fourth driving signals for respectively controlling duty cycles of the first, second, third, and fourth switches so as to make the switching regulator circuit enter a buck mode, a boost mode, or a buck-boost mode,
• wherein the control logic make the switching regulator circuit enter the buck-boost mode by periodically switching the first through the fourth switches in accordance with the sequence of AC to AD to AD to BD, starting from said AC, so as to make only two switches among the first through the fourth switches turn on and off only once during a single switching cycle of the switching regulator to adjust the regulated output voltage,
• said AC means the first and the third switches are both turned on, and the second and the fourth switches are both turned off,
• said AD means the first and the fourth switches are both turned on, and the second and the third switches are both turned off, and
• said BD means the second and the fourth switches are both turned on, and the first and the third switches are both turned off.