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Shrinking portable power management

Posted: 01 Jul 2006 ?? ?Print Version ?Bookmark and Share

Keywords:John McGinty? Micrel? portable electronics? multimedia functions? power management?

processors that drive multimedia functions are decreasing power consumption by reducing the voltage from which they operate.

Today's portable electronics encompasses advanced multimedia functions, and the amount of processing power inside the mobile phone is increasing exponentially.

The processors that drive this content are decreasing power consumption by reducing the voltage from which it operates, with I/O voltages at 1.8V and core voltages quickly approaching the 1V level.

The output voltage from the common power source for portable electronics, a single-cell Li-ion battery, is approximately 3.6V. Thus, to achieve high conversion efficiency and good battery life, multiple DC/DC converters need to be used. But DC/DC converters require larger external components compared with its power-conversion predecessor, the LDO regulator.

The problem with using multiple converters is the external components. The highest-efficiency converters use a magnetic-based conversion technique in which energy is stored in an external inductor and transferred to the load, achieving 90 percent efficiency or more.

The physical size of the inductor consumes board areamuch more than the ICs occupy. Key to reducing inductor size is increasing the switching frequency of the converter; inductor value is typically inversely proportional to the switching frequency of the converter.

The inductor must not saturate during switching operation to ensure efficient energy storage and transfer. A saturated inductor results in a direct DC path and no energy storage, defeating the whole purpose of the switched-mode converter. The inductor must be large enough to operate with a specific frequency converter and not saturate.

The switching frequency sets the minimum practical inductor value. A 1MHz inductor has a minimum inductor value that is higher than a 2MHz inductor, although their sizes are similar. The size of the inductor is proportional to its inductance value. Thus, as the inductance value decreases, so does the inductor's physical size. But the required inductor value is not determined entirely by the switching frequency. It is limited by the switching frequency to a minimum value, but the rest of the value is set by the compensation and loop design of the converter itself. Check the data sheets of the converter thoroughly before making final component selection.

If the switching frequency of the DC/DC converter is increased, the total solution size can be decreased. With a switching frequency of 8-10MHz, the inductor value can be reduced to 0.47H from the typical 4.7H, which is currently used in mainstream power-management ICs and discrete converters. Micrel's MIC2285 is an example of a converter operating at that frequency and working with a 0.47H inductor. The inductor size is reduced from a 3mm2 surface area down to 1-by-1.2mma size reduction of more than 85 percent.

The higher switching frequency allows the use of chip inductors, rather than standard wire-wound inductors; they also reduce the height to less than 1mm, a critical feature in today's ultrathin models.

The value of space is increasing as handheld devices become portable media communication centers, driven by the integration of digital, audio and video processing into one device. Innovative technology such as ultrahigh operating frequencies will drive the necessary performance while maintaining the small form factors we expect.

- John McGinty
Strategic Applications Engineer, Micrel Semiconductor

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