Many of today's MP3 players, multimedia players, digital video recorders, and other portable electronic devices have found the micro hard disk drive to be the optimum device for memory storage requirements.
Given the Lithium-ion battery voltage range of 2.5V to 4.2V and the typical micro hard disk drive 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/up (buck-boost) converter.
A buck converter (Figure 1) provides the most efficient solution with the smallest external components, but drops out near 3.3V. At this point, the converter transitions to 100% duty cycle operation and tracks the battery voltage. A buck- boost solution has the advantage of regulating the output voltage over the full Lithium-ion voltage range, but suffers from lower efficiency and a larger total footprint. Additionally, most Lithium-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.
Theory
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 (Figure 2). 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% 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?
Figure 1: Lithium-Ion to 3.3V 1.3MHz Step-Down Buck Converter
Figure 2: Lithium-Ion to 3.3V 1.5MHz Buck-Boost Converter (Buck: Q1 and Q2 Switching, Q4 On; Boost: Q3 and Q4 Switching, Q1 On)
