Electronic Design

Power: LDO Voltage Regulators

Sponsored by: NATIONAL SEMICONDUCTOR

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 (VREF). If the output voltage increases relative to VREF, the error amplifier changes the pass-transistor's output to maintain a constant output voltage (VOUT).

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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