A substantial percentage of regulator requirements involve stepping down the primary voltage. Although linear regulators can do this， they cannot achieve the effi ciency of switching based approaches1. The theory supporting step-down （“buck”） switching regulation is well established， and has been exploited for some time. Convenient， easily applied ICs allowing implementation of practical circuits are， however， relatively new. These devices permit broad application of step-down regulation with minimal complexity and low cost. Additionally， more complex functions incorporating step-down regulation become realizable.

　　Basic Step Down Circuit

　　Figure1 is a conceptual voltage step-down or “buck” circuit. When the switch

　　closes the input voltage appears at the inductor. Current fl owing through the inductor-capacitor combination builds over time. When the switchopens current fl ow ceases and the magnetic fi eld around the inductor collapses. Faraday teaches that the voltage induced by the collapsing magnetic fi eld is opposite to the originally applied voltage. As such， the inductor’s left side heads negative and is clamped by the diode. The capacitors accumulated charge has no discharge path， and a DC potential appears at the output. This DC potential is lower than the input because the inductor limits current during the switch’s on-time. Ideally， there are no dissipative elements in this voltage step-down conversion. Although the output voltage is lower than the input， there is no energy lost in this voltage-to-current-to-magnetic fi eld-to-current-to-charge-to-voltage conversion. In practice， the circuit elements have losses， but step-down effi ciency is still higher than with inherently dissipative （e.g.， voltage divider） approaches. Figure 2 feedback controls the basic circuit to regulate output voltage. In this case switch ontime （e.g.， inductor charge time） isvaried to maintain the output against changes in input or loading.