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The evolution of the buck-boost DC/DC regulator.

DC/DC buck (step-down) converters and boost (step-up) converters address a unidirectional voltage conversion of always regulating a voltage less than or more than the input supply, respectively. However, the question of how to regulate a voltage accurately if the input supply varies above, below or equal to the output voltage, i.e. bidirectional, has been addressed by different and more complex DC/DC topologies.


For example, a flyback topology is used in many battery backup systems where the battery voltage varies depending on the state of charge and charge capacity. A buck-boost is also a valuable solution in systems with poorly regulated input supplies that can rise above or fall below the value of the desired output voltage.

A Brief History

The early approach in designing a DC/DC regulator that could regulate a fixed output independent of the relative value of the input voltage whether [V.sub.IN] is higher or less than [V.sub.OUT] was the flyback. Weathering the times for many years, flyback is still used because most analog engineers are very familiar with its design and aside from a good understanding of magnetic and MOSFET behaviors, the design is fairly simple. However, a flyback regulator for some DC/DC conversion may require a special transformer design, and its efficiency is often low for most of today's applications. This leads to the next option: the SEPIC.

SEPIC is an acronym for single-ended primary inductance converter. The basics of a SEPIC design rely on a DC/DC boost converter IC, a coupled inductor, or a transformer. High power SEPIC designs will require external power MOSFETs that are often large because they have to withstand high voltage transients and provide low [R.sub.DS(on)]. The coupling inductor is often not an off-the-shelf inductor, and its construction plays an important role in the performance of the power supply design. Moreover, a SEPIC, and to some extent the flyback, have an operating efficiency range between 67 percent to 86 percent, depending on many factors such as the conversion ratio, choice of magnetics, capacitors, and MOSFET. Layout is also very crucial in the stability and heat management.

The salvation for applications that demanded high power buck-boost but for which flyback and SEPIC was unacceptable because of low efficiency and heat dissipation has been the single inductor four-switch buck-boost controller. The circuit and architecture could easily meet 90 percent to 95 percent efficiency at 60W (12V, 5A) output. The solution meets the power requirement of most applications and delivers it very efficiently while transition from different mode operation, from buck to boost or boost to buck, occurs seamlessly. With four MOSFETs, one off-the-shelf inductor without coupling, the synchronous DC/DC controller, compensation circuitry and a good knowledge of layout, one could implement this smaller and higher efficiency design. Figure 1 shows a comparison between a SEPIC and this buck-boost synchronous solution.


The Search for An Elegant Buck-Boost Device

The synchronous four-switch buck-boost idea allows designers to deliver higher power while dissipating less heat than flyback or SEPIC. The magnetic is also more readily available: no special inductor or transform winding is necessary. However, some of us may be intimidated with this circuit because it needs approximately 24 components. Or we may not have analog expertise or must finish a complicated system without the time to wrestle with the power supply section of the project.

A new generation of highly integrated devices has emerged to address this issue. For example, Linear Technology's LTC3780 buck-boost circuit comes in a 1 5 mm X 15 mm X 2.8 mm LGA package that weighs 1.5g. It only needs an off-the-shelf inductor, a resistor to set the output voltage, a sense resistor and input output bulk capacitors. The MOSFETs, compensation circuitry and the sophisticated DC/DC controller are all integrated in a protective plastic mold package. Figure 2 shows the comparison between this DC/DC [micro]Module buck-boost circuit and a discrete LTC3780 design.



Creating a buck-boost circuit design can be a daunting task. Besides knowing the mathematics behind the selection of the value of the component, designers must understand the physics such as the effect of the layout, ground planes, PCB via, soldering and thermal management. A high power and high efficiency buck-boost that is easy to design and requires minimal amount of power supply design can alleviate many digital circuit designers' worries and concerns.

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by Afshin Odabaee

Linear Technology
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Title Annotation:Low Power Design
Author:Odabaee, Afshin
Publication:ECN-Electronic Component News
Date:Mar 1, 2008
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