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EDNE MAY 2015

Power conversion MicroSIP unwrapped; Anatomy of a complete power supply as simple as an LDO By Thomas Schaeffner, Texas Instruments About 10 years ago, a fully integrated step-down converter for an output current of 600 mA was running at a switching frequency of 750 kHz, used a 10 μH inductor and was packaged in leaded plastic packages such as MSOP10. The package itself needed a space of 3 x 5mm, partially because it had leads on two sides. Passive components such as the ceramic capacitors were 1206 size which measure 3 x 1.5mm. The inductor added another 4.5 x 4.5mm and was, with a height of 3 mm, by far the largest component in size and volume. A complete power solution required a board space Inters of 170 mm². Shortly afterwards, QFN packages became standard as they did not need extra space for their pins because they are located under the package and therefore allowed to minimise the space needed for a given pin count. In addition, the PowerPad, an exposed pad on the bottom side of the package, provides a low thermal resistance to the PCB and helps to increase the output power. Along with the progress in passive components, this allowed for the package size to be cut by a factor of 14, down to 28 mm². A contributor to this progress was the increase in the switching frequency to the 2 MHz range. The inductor became much smaller as the inductance required was reduced to 2.2 μH. As the physical size of an inductor scales with its inductance, a lower inductance means a smaller device. The higher switching frequency also allows the design of power supplies with better electrical characteristics, such as transient response. For a smaller inductance, the Figure 1. Device and package evolution 25 EDN Europe | MAY 2015 www.edn-europe.com


EDNE MAY 2015
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