Product of the Year: LDO Redesign Creates a More Versatile Regulator

Oct 1, 2007 12:00 PM
By David Morrison, Editor, Power Electronics Technology

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Among the many hundreds of new power components introduced each year, there are numerous devices with intriguing performance improvements, novel combinations of functions or advances in packaging. This continuing wave of technical innovation makes selecting a single product for special recognition a particularly daunting challenge.

Ultimately, the device selected for Product of the Year award must offer a refreshing combination of novelty and usefulness. Linear Technology's LT3080, an adjustable 1.1-A low-dropout regulator (LDO), appears rich in both qualities. The LDO's uniqueness lies mainly in its ability to be paralleled for current scaling or heatspreading, and in its ability to generate an output that's adjustable down to 0 V. These features, combined with the LT3080's wide input-voltage range, make the LDO unusually versatile, giving designers new options for board-mounted power conversion and for reducing the number of part numbers they stock.

For users, the ability to parallel regulators eliminates worries over whether the devices will current share properly. Engineers who parallel conventional linear regulators may do so at their own risk, because chip vendors don't usually guarantee such operation.

In contrast, multiple LT3080s can be paralleled simply by tying together the corresponding adjustment pins and output pins. So, while a single LT3080 is rated at 1.1 A, multiple LDOs may be paralleled to deliver a few amps (or more) of current, while staying with all-surface-mount assembly.

It's not so much that a single surface-mount LDO can't deliver more than 1 A, but rather that the pc board often becomes the limiting factor in how much heat can be pulled out of the package. Using the LT3080 gives designers an LDO option that doesn't require heatsinks.

The LDO approach is particularly beneficial in systems with multiple low-voltage supply rails. In such systems, LDOs can step down an existing supply voltage to a lower value with reasonable efficiency, while achieving the benefits of linear regulation (simplicity, low cost, low noise and fast response). In such systems, the LT3080's ability to generate outputs down to zero should become increasingly useful as the voltage requirements for logic devices continue to fall.

The LT3080's wide input-voltage range (1.2 V to 36 V)is another quality that contributes to its usefulness, particularly in automotive and other applications where higher-voltage supply rails and transients are encountered. Another feature is the ability to adjust the LT3080's output voltage using a single resistor, rather than the two normally required.

In designing the LT3080, Linear broke away from the conventional LDO architecture, using a current source reference rather than a voltage reference. Linear Technology Vice President Bob Dobkin, who conceived this LDO design years ago, comments, “Generating a stable current in an IC is very difficult. Current must be stable with temperature and changes in voltage and power dissipation. It requires carefully matched internal voltages to the TC [temperature coefficient] of the resistor, so that the current that comes out of [the LDO's adjustment pin] has low TC.”

Designing the output amplifier stage was also a challenge. “The offset between the adjustment and the output of the regulator is a millivolt,” says Dobkin. “That's about the same as a pretty good op amp.” The low offset allows paralleling of devices. According to Linear Technology's Tony Armstrong, there were no semiconductor process limitations to overcome in developing the LT3080. The part is fabricated in a fairly standard, 2-µm, 40-V bipolar process.

The company plans to extend its use of the LT3080's design by offering various current ratings in 2008. These new devices will also expand packaging options. The LT3080 comes in a 3-mm × 3-mm DFN, an MSOP, a SOT-223 or a TO-220. Future models will offer packages as small as 2-mm × 2-mm DFN.

For more information on the LT3080, see “LDO Breaks Voltage Barriers, Allows Paralleling,” PETech, July 2007.