Choosing between a positive or negative buck topology in a switching regulator design

Hector Arroyo

April 25, 2008

Hector ArroyoApril 25, 2008

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.

This article analyzes the architecture 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.

Figure 1: Shown is the basic architecture of a negative buck topology.

Negative buck topology
Figure 1 above shows the basic architecture of a negative buckswitching converter. 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 negative-channel FET (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 usually have this function built in. In a negative buck, an NFET used as a high-side pass device also requires a gate-drive voltage more positive than the system's input (-Vin). In this case, since this input voltage is the most negative voltage in the system, no special circuitry is needed.

All other voltages, including the output, are "higher" (more positive), with the converter ground being the most positive voltage in the system. Under these circumstances, a low-side FET pulse-width modulator control IC (such as a boost/flyback regulator or controller) can be used to implement the converter.

A variety of ICs may be used to implement negative buck converters, including controllers and integrated monolithic regulators with low side NFETs. Monolithic ICs provide simplicity, ease of implementation and lower component count. Controllers offer greater flexibility when larger output currents are needed, and when there's a need to optimize for efficiency and thermal dissipation.

Figure 2: This negative buck topology uses a monolithic LM5001 boost/flyback regulator.

Figure 2 above shows a simplified diagram of a 3.1-75V input- voltage-range boost/ flyback regulator in a negative buck topology with a built-in 75V, 1A NFET.

In a regular boost application, it will put out a gate-drive voltage to its built-in pass N-channel MOSFET a few volts above ground in order to turn it on. In a negative buck application, the gate drive will still put out a gate voltage a few volts above the IC's ground pin, which in this case is tied to the system's input voltage (-Vin) and will yield the needed results.

Different from a regular boost, but the same as a regular buck, peak IC-switch current in Figure 2 is the same as peak inductor/output current, thus allowing a 1A boost IC to be used for output currents up to 1A. Other regulators with different ratings would be used for higher or lower switch currents. If a controller is preferred, it would be used in a similar configuration to the one in the figure.

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