By Karl R. Volk
Modern portable consumer electronics are integrating so many features that the latest incarnations often become hard to classify. While more features may improve sales, system designers are challenged to meet the consumer's expectations for small size and long battery life. Each new feature requires extra space and processing horsepower, leaving less room for the battery and increasing the power supply demand for higher output current with greater efficiency in a smaller space.
As recently as one or two years ago, most handheld electronics typically used zero or one step-down (buck) converter and many low-dropout (LDO) linear regulators to power the various functional blocks. This worked well because popular processors were typically powered at 3.0 to 3.3 V where the LDO is suitably efficient with a single-cell lithium-ion (Li-Ion) battery input.
However, as processing demands increased and IC process technology migrated to smaller submicron geometry, popular core voltages dropped to 1.8, 1.5, 1.3 and even 0.9 V. On top of that, typical I/O voltages have dropped from 3.3 to 2.5 or 1.8 V. At these low output voltages, the LDO becomes very inefficient and produces significant heat, negating some of the advantages of the low-voltage core and I/O.
To compound the need for low supply voltages, many systems now rely on multiple processors, as shown by the cellphone and PDA combination, which generally has a baseband processor and an applications processor, each requiring individual power. Today's popular camera modules still tend to run on an LDO, but the associated graphics processor may also need a low voltage. Consequently, modern multifunction designs often employ multiple buck converters. It is not uncommon to find three buck converters on the PCB. Often, the power-management custom IC (PMIC) is integrating one or more buck converters, but with every new feature, there is the potential for yet another.
Until recently, the cost of a buck converter could not only be counted in dollars, but also in substantial pc-board real estate. The typical small buck converter of only three years ago was packaged in 15-mm3 molded small-outline package (MSOP) and switched at 1 MHz or less, requiring a big external inductor and usually large tantalum capacitors.
Today's 1-MHz buck converter is far improved with 9mm3 dual flat no-lead (DFN) package, ceramic capacitors, and newer, smaller inductors. However, it is still far larger than the typical LDO. The key to solving the size problem is that modern, submicron BiCMOS mixed-signal processes enable smaller power supply ICs and faster switching frequencies for smaller external components. Today, many manufacturers offer 2 MHz or higher in a small package. Some 4-MHz buck converter solutions may be nearly as small as an LDO.
Because process advancements benefit both the 1- and 4-MHz buck converters, the 4-MHz buck still falls short of the 1-MHz buck efficiency. High-speed switching causes more switching losses and capacitive losses in the converter, while the tiny inductors tend to have more magnetic field core-loss. However, the efficiency difference is not so great, especially compared with the LDO's low 41 percent efficiency.
This leaves system designers with a choice of 1) smallest size, 2) highest efficiency, or 3) small size and high efficiency, allowing a trade-off between battery life and small physical size. Since the high-frequency buck converter offers a dramatic efficiency improvement without much increase in size, it is becoming the favored solution in multifunction portable consumer handhelds. And looking to the future, because the buck converter produces less heat than the LDO, it could potentially displace the LDO as the smallest solution.
Karl Volk is a corporate applications engineer for Maxim Integrated Products (Sunnyvale Calif.).