LDO Breaks Voltage Barriers, Allows Paralleling

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

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Imagine a low-dropout linear regulator (LDO) that can be produce an output that's adjustable all the way down to 0 V, and that can be easily paralleled for current scaling and heat spreading. Then imagine that the device requires only a single resistor for setting the voltage plus two small ceramic capacitors to complete the design. Those qualities, along with the new architecture that enables them, distinguish Linear Technology's LT3080 adjustable 1.1-A LDO from other LDOs on the market.

Although it has a wide-input-voltage range (1.2 V to 30 V), the LT3080 targets applications where one voltage supply needs to be stepped down to another low-voltage supply at relatively low current levels. For example, the LDO can step down a 1.8-V supply to 1.3 V, offering efficiency that's on par with switchers, while also requiring fewer components and less pc-board area. Moreover, the LDO design typically costs less than that of a switching regulator.

Like other linear regulators, the LT3080 can be used where low noise is a must. So for designs that cannot tolerate the ripple produced by a switcher, the new LDO can be deployed. Although existing LDOs might serve this purpose (depending on output voltage), those ICs may require heatsinking.

In contrast, multiple LT3080s can be paralleled simply by tying their adjustment pins and output pins together (see the figure). No external components are needed for paralleling units, but a small length of pc-board trace is required on each output to provide ballast (about 10 mΩ) to ensure current sharing between regulators.

As many as 10 LT3080s might be paralleled to achieve a 10-A supply that requires no heatsinking and is completely surface-mount. However, Bob Dobkin, vice president of Linear Technology, points out that 1 A to 3 A is typically the range where the LT3080 is most useful, because of the cost advantage it offers versus switchers in this range.

In designing the LT3080, Linear broke away from the conventional LDO architecture. Dobkin describes the design as basically a precision op amp capable of operating down to 0 V combined with a precision current source, “all disguised as a low dropout regulator.”

Using a current-source reference rather than a voltage reference presented design challenges. “Generating a stable current in an IC is very difficult. Current must be stable with temperature and changes in voltage and power dissipation. No other IC regulators use current as the reference source because it is difficult to do. It requires carefully matched internal voltages to the TC [temperature coefficient] of the resistor so that the current that comes out of it [the LDO's adjustment pin] has low TC.”

Designing the output stage was also a challenge. “The offset between the adjustment and the output of the regulator is a millivolt. That's about the same as a pretty good op amp and it's being done in a regulator that's handling tens of watts.” The low offset is needed to allow paralleling of devices.

Dobkin notes that designing the LT3080 required great attention to detail. The reference circuit operates internally with 1 V across it. The minimum input-voltage requirement is basically that 1 V plus the saturation voltage of a transistor current source that drives the circuit. “To get those kinds of voltages, the circuitry cannot have two emitter-base voltages in series,” says Dobkin. “Not being able to have two transistors in series makes the design a lot more difficult.”

Despite the challenges, Dobkin comments that there's been little compromise in the key LDO specifications. Dropout voltage is specified as 300 mV at 1.1 A, although that performance requires a second low-current-input voltage 1 V above the output. Alternatively, a single supply can be used for a dropout of 1 V. Accuracy of the reference is 1%, load regulation is approximately 2 mV, and line regulation is <0.001%/V. Minimum load current is 1 mA.

The device is offered in an 8-lead, 3-mm × 3-mm DFN, an 8-lead thermally-enhanced MSOP, or a 3-lead SOT223. The device is also offered in a TO-220 that's heatsinkable for higher power dissipation. In quantities of 1000, unit pricing starts at $1.88, $1.94, $1.81 and $2.20 each, respectively. For more information, see www.linear.com.

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