Power: LDO Voltage Regulators

Aug. 23, 2004

What's an LDO? The low-dropout (LDO) voltage regulator offers a much smaller minimum required voltage (the dropout voltage) between its input and output voltage levels than other types of regulators. The LDO's main components are a power semiconductor (pass transistor), error amplifier, and voltage reference (see the figure). One input to the error amplifier, set by resistors R1 and R2, monitors a percentage of the output. The other input is a stable voltage reference (V_REF). If the output voltage increases relative to V_REF, the error amplifier changes the pass-transistor's output to maintain a constant output voltage (V_OUT).What does low dropout mean? Low dropout refers to the smallest difference between the input and output voltages that allows the IC to still regulate the output voltage. That is, the LDO device regulates the output voltage until its input and output approach each other at the dropout voltage. Ideally, the dropout voltage should be as low as possible to minimize power dissipation and maximize efficiency. And because of this low dropout voltage, the LDO extends battery life by permitting the battery to be discharged all the way down to a few hundred millivolts of the desired output voltage.Why use an LDO instead of a switching regulator? The major advantage of an LDO IC is its relatively "quiet" operation because it does not involve switching. In contrast, a switch-mode regulator typically operates between 50 kHz and 1 MHz, which can produce EMI that affects analog or RF circuits. LDOs with an internal power MOSFET or bipolar transistor can provide outputs in the 50- to 500-mA range. The LDO's low-dropout voltage and low quiescent current make it a good fit for portable and wireless applications.

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About the Author

Sam Davis 2

Sam Davis was the editor-in-chief of Power Electronics Technology magazine and website that is now part of Electronic Design. He has 18 years experience in electronic engineering design and management, six years in public relations and 25 years as a trade press editor. He holds a BSEE from Case-Western Reserve University, and did graduate work at the same school and UCLA. Sam was the editor for PCIM, the predecessor to Power Electronics Technology, from 1984 to 2004. His engineering experience includes circuit and system design for Litton Systems, Bunker-Ramo, Rocketdyne, and Clevite Corporation. Design tasks included analog circuits, display systems, power supplies, underwater ordnance systems, and test systems. He also served as a program manager for a Litton Systems Navy program.

Sam is the author of Computer Data Displays, a book published by Prentice-Hall in the U.S. and Japan in 1969. He also authored the book Managing Electric Vehicle Power. He is also a recipient of the Jesse Neal Award for trade press editorial excellence, and has one patent for naval ship construction that simplifies electronic system integration.

You can also check out additional articles on his other author page.