New topology reduces inductor height in ultra-small 12 V step down converter
The topology implemented byTexas Instruments for the TPS54A20 SWIFT synchronous DC-DC converter enables high-frequency operation at up to 5 MHz per phase without special magnetics or compound semiconductors. This gives designers 8 V to 14 V input and 10 A output applications with standard inductors less than 1.2mm high
The TPS54A20 uses a unique two-phase, series-capacitor DC/DC buck topology that merges a switched-capacitor circuit with a multiphase buck converter. This capacitive conversion technology enables efficient, high-frequency operation and measures as much as seven times smaller than conventional converters.
“The topology has been around for a while but we are the first to integrate it into a chip,” said Pat Hunter, worldwide analog content manager at TI. “The series capacitor is the key to the design and that’s where we get the efficiency from – it’s a simple off the shelf capacitor. We can run up to 2 to 5 MHz per phase so the effective ripple is 4 to 10 MHz so it truly is a 10MHz converter.”
“This means we can use little chip inductors that are off the shelf, so we have stepped over the inductor boundary,” he said. “It used to be the inductor that was the limiting factor [for the height] and now it’s the capacitor.”
The MOSFETs are packaged in a small quad flat no-lead (QFN) packaging, and combined with 2- to 5-MHz inductors enable a 131-mm2 module less than 2mm high. This is half the area of similar 500 kHz DC/DC converter designs. This allows placement of a 10 A voltage regulator on the back side of a printed circuit board (PCB) for industrial servers or other space-constrained applications.
TI is developing a 15 x 9 mm overmoulded module that is 2.3mm high using the chip. “The only thing required will be one Vset resistor, the bottom feedback resistor, and that will be later this year,” he said. This also opens up the potential for the inductors and capacitors to be integrated into a multi-level ceramic carrier (MLCC) that will reduce the size even further.
The converter is packaged in a 20-pin, 3.5-mm-by-4-mm-by-1-mm thermally enhanced HotRod QFN package and priced at US$3.25 in 1,000-unit quantities. A 4 MHz, 10 A synchronous dual-phase converter evaluation module is available as well as the PSpice transient models.
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