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Buck boost vs. buck converter battery life for portable 3.3V micro HDD applications

Posted: 12 Nov 2009     Print Version  Bookmark and Share

Keywords:buck converter battery  buck boost converter  micro HDD 3.3V 

Many of today's MP3 players, multimedia players, digital video recorders and other portable electronic devices have found the micro HDD to be the optimum device for memory storage requirements.

Given the Li-ion battery voltage range of 2.5V to 4.2V and the typical micro HDD voltage of 3.3V, an obvious dilemma associated with this application is whether to use a low dropout DC/DC step-down (buck) converter or a step-down/step-up (buck-boost) converter.

A buck converter provides the most efficient solution with the smallest external components, but drops out near 3.3V. At this point, the converter transitions to 100 per cent duty cycle operation and tracks the battery voltage. A buck-boost solution has the advantage of regulating the output voltage over the full Li-ion voltage range, but suffers from lower efficiency and a larger total footprint. Additionally, most Li-ion batteries have a plateau from 3.5V to 3.6V and very little charge below the plateau, limiting the usefulness of the buck-boost converter and its wide input voltage range.

The buck-boost converter uses a four-switch configuration, operating as a buck, boost or buck-boost converter. It has four power switches, only two of which switch at any given time, with a third that remains on continuously throughout the switching period. For a battery voltage greater than 3.6V, the buck-boost operates as a buck converter while, for an input voltage less than 3.6V, it behaves similar to a buck-boost or boost converter. Below 3.6V, the buck-boost converter input current increases beyond the load current, while the buck converter enters the 100 per cent duty cycle dropout where the input current is limited to the load current and the output voltage tracks the input.

So, which converter will provide the best battery life for this application?

View the PDF document for more information.

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