Buck boost switching regulator with duty cycle limiting

In a buck-boost, switching regulator, separate signals are developed to control duty cycles of regulator switches (10). Control exercised by a pair of feedback-determined reference signal voltages Vcg, V°b through which a ramp signal (from 11) is swept at inputs of separate comparators (12, 13), comparator outputs control respective switches. A predetermined maximum voltage difference is permitted between the reference signal voltages. The reference used to control switching of one end of the regulator inductance (part of 16) is limited to a minimum voltage corresponding to a minimum duty cycle greater than 0% while the other reference is used to control the other end of that inductance and is limited to a maximum voltage corresponding to a minimum duty cycle less than 100%. A current limiting (20) feedback from one of the switches is operated in response to current in excess of a predetermined level to force the ramp signal to a level corresponding to 0% duty cycle for both switches at the same time.