step down switching converter

Posted on Feb 6, 2014

High-efficiency step-down switching regulators for positive voltages are very common, however negative step-down switching regulators (negative voltage in, negative voltage out, common ground) are not as well known, even though they are often needed. Although they are not difficult to set up, literature on how to build them is rather scarce. Thisarticle analyzes the architecture

step down switching converter
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and detailed operation of the negative buck topology. It will also discuss actual circuit implementations for the topology, from a system perspective down to the building of the needed circuit blocks, and include examples on how to build a voltage translator circuit, a key block in implementing a negative buck regulator using readily available boost ICs. (Note: In the figure, while it is common to draw positive rails at the bottom of circuits in schematic diagrams and, based on it, negative buck topologies show ground (most positive rail) on the top in many cases. Here, ground was drawn at the bottom with the circuit`s Vin and Vout at the top on purpose, to reflect the simplicity and similarity of this topology compare to the positive buck. ) Like a positive buck design, it has a high-side pass device between input and output, an LC output filter, and a catch diode. The two big differences are the gate drive needed in the control IC and the feedback circuitry. In a positive buck, a typical NFET used as a high-side pass device requires a gate-drive voltage higher (more positive) than the systems input voltage (Vin) in order to be turned on. Since the input voltage is the most positive voltage in the system already, special circuitry is needed to generate an even higher voltage. Positive buck ICs have usually this function built in. In a negative buck, an NFET used as a high-side pass device also requires a gate-drive...

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