POL Breaks New Ground with Integrated Magnetics

Oct 1, 2004 12:00 PM
By David Morrison, Editor, Power Electronics Technology

Power IC manufacturers have been making steady progress in the development of increasingly integrated power supply controllers, particularly those aimed at nonisolated, point-of-load converter (POL) applications. Some of these dc-dc controllers now include all of the silicon — control circuitry, gate drivers and power MOSFETs — needed to build a synchronous buck converter. But progress in shrinking the POL further has been stymied by the inductor, a large discrete that has long resisted integration.

However, Enpirion's announcement earlier this year of an IC-style POL with a co-packaged inductor demonstrated the feasibility of using microelectromechanical system (MEMS) technology to build POLs with integrated magnetics. In May, the power supply developer exploited innovations in power controller design, CMOS process technology and MEMS magnetics to build a 10-W buck converter in a 12.5-mm × 8.1-mm DFN package.

With its inductor in package, the POL requires only a few external ceramic capacitors and a resistor to complete the design. The EN5330 dc-dc converter delivers up to 3 A of output at a selectable voltage ranging from 0.8 V to 3.3 V, while operating from a 2.5-V to 6.5-V input.

Although the EN5330 co-packages the inductor and dc-dc converter chip in a side-by-side arrangement, the underlying technology lays the groundwork for more ambitious integration plans. The company is working toward integration of the converter chip and the MEMS inductor on a single piece of silicon.

For now, the co-packaging of the inductor and dc-dc controller achieved in the EN5330 represents an impressive technical feat. To construct the MEMS inductor, the company developed a proprietary iron-based alloy, which it used to plate a copper spiral on a silicon substrate. The substrate is positioned between two halves of a magnetic core, which results in excellent high-frequency and saturation characteristics. This inductor performs well at frequencies up to and beyond 10 MHz.

To make this inductor small enough to fit in the DFN package, Enpirion pushed the typical switching frequency of its dc-dc converter chip to 5 MHz. To do so, while still achieving efficiencies approaching 90%, required innovations in circuit design and CMOS process technology. One innovation was the development of a power MOSFET based on a modified RF transistor. The reduced parasitics associated with this power switch enabled a 10-fold increase in switching speed versus that of conventional designs.

The company has been issued patents for this power MOSFET, its control algorithm, its integrated magnetics and its process technology. Pushing the frequency to 5 MHz not only shrunk the magnetics, but also greatly reduced external capacitance requirements.

As a result of the EN5330's high degree of integration, a complete dc-dc converter design occupies 70% less area and uses 60% fewer parts than comparable designs. The entire dc-dc converter can be placed in as little as 135 mm2 of single-sided or 102 mm2 of double-sided board space.

The EN5330 is an analog device using voltage-mode control. However, the high switching speeds achieved by the company's power MOSFETs enable very fast transient response. That response lends itself to the application of dynamic voltage scaling, a technique that could be used to closely match the power generated by the converter to that required by a microprocessor. To this end, the company is developing a version of its converter using digital control to perform dynamic power management (DPM) for low-power, portable applications.

Already sampling, the EN5330 begins production in the fourth quarter of this year. The EN5330 represents the first member of the company's iPOWER family with other devices expected in the coming months.

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