Latest Step in DC-DC Integration Leaves Tinier Footprints on PCBs

May 9, 2007 4:52 PM
by David Morrison, Editor, Power Electronics Technology

In the quest to both shrink and integrate dc-dc converters for portable applications, chip makers are increasing the number of buck converters and low dropout regulators they can squeeze into a tiny surface-mount package. Two new ICs from Linear Technology and Maxim Integrated products demonstrate the current state of the art, packing four dc-dc channels into 3-mm x 3-mm footprints.

Linear Technology’s LTC3544 combines four independent synchronous buck regulators in a 3-mm x 3-mm QFN package (see the figure). These four buck converters are rated at 300 mA, 200 mA, 200 mA and 100 mA of continuous output. Using constant-frequency current-mode control, the LTC3544 steps down a 2.25-V to 5.5-V input to output voltages as low as 0.8 V. This input range accommodates the popular battery types, while the output range enables the part to power the latest generation of low voltage DSPs and microcontrollers. A high, 2.25-MHz switching frequency helps shrink the external passives, even permitting use of inductors less than 1 mm in height, yet still allows for good efficiency.

According to the vendor, the LTC3544 represents a significant advance in integration for buck converter ICs. “Nobody packs four synchronous buck converters in a 3-mm x 3-mm footprint,” says Tony Armstrong, product marketing manager of Linear’s Power Products Group, who notes that the most comparable devices until now have been duals such as Linear’s own LTC3547 in a 2-mm x 3-mm DFN.

Another point of reference is Texas Instruments’ TPS62400, a 400-mA/600-mA dual stepdown converter in a 3-mm x 3-mm QFN. Although this is a dual-channel converter, it produces higher output current levels than the Linear part. Also note that, like the ‘3544, it switches at a fixed 2.25 MHz. (For more on this part, see “Stepdown Converters Support Dynamic Voltage Scaling.”)

In terms of functionality alone, there are some highly integrated power management ICs (PMICs) developed specifically for portable applications and these devices can integrate many channels of dc-dc conversion. However, these are typically much larger chips than the LTC3544 and they have much higher pin counts. (For some examples, see “Portable Power Management Inspires Mega Integration.”) With the 16-lead LTC3544 and other devices, Linear takes the building block approach in addressing portable power requirements, offering modest levels of integration combined with high performance.

Figure: With its integration of four synchronous buck converters in a 3-mm x 3-mm QFN and its use of tiny passives, the LTC3544 provides a compact power-conversion building block for portable applications.

In the case of the ‘3544, that high performance includes high efficiency and low noise. For example, the converters achieve efficiency on the order of 92% to 93% when stepping down a 3.6 V supply to 1.8 V. In addition, efficiencies up to 95% are possible. In terms of standby operation, the LTC3544 offers Burst Mode operation where quiescent current is reduced to 70 µA total for all four channels under no load conditions.

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