Self oscillating control of a synchronous DC-DC buck converter

This paper presents a control scheme for synchronous DC-DC buck converter, which is designed to operate at low output voltage and high efficiency typically required for battery-operated systems. The converter is designed for equipment, which demand a high current only for a short duration. Such systems are mobile phones, intelligent wireless sensors and backlit displays which need to be in the active mode for only a short period of time and go to a standby mode for the rest of the time to conserve the battery power. With decreasing size of transistors and lower leakage loss, systems are increasingly being operated at lower supply voltages. Efficient power management techniques are needed, which must be able to regulate the output at lower voltages under all loading conditions. To make the DC-DC converter efficient at lower voltage, synchronous converter is an obvious choice because of lower conduction loss in the diode. High efficiency under the given loading conditions is typically achieved by operating it in the so-called pulse-skipping mode (PSM), which is a special form of discontinuous mode that maintains a constant value of the peak inductor current. Control of synchronous DC-DC converter in PSM is difficult because of one has to control both the high-side and low-side switches avoiding shoot-through and a negative current through the inductor. One needs to incorporate an appropriate dead-time between the two switches, which needs to be carefully designed so that the overall performance is not unduly affected. The control scheme typically requires an oscillator or a current sensing mechanism, each of which has some associated disadvantages. This paper presents a self-oscillating control scheme that tries to overcome the difficulties in the existing approaches. A design and its implementation have been carried out in a 1.5u BiCMOS process and both simulation and experimental results are presented.