What is a low-dropout regulator? LDO selection explained

A low-dropout regulator, or LDO, is a linear voltage regulator that maintains a lower output voltage with relatively little voltage difference between its input and output. It controls a pass device using feedback. Choosing an LDO requires checking dropout at the intended load, output accuracy, noise, capacitor requirements and heat dissipation.

LDOs are often useful for low-power rails or for post-regulating a switching supply. A small input-to-output difference can make them efficient, while a large voltage drop at high current can create substantial heat. Low dropout describes a headroom requirement, not a guarantee of low dissipation in every circuit.

How an LDO regulator maintains its output voltage

The feedback network senses the output and compares it with an internal reference. An error amplifier adjusts the pass device to correct changes in supply or load. A fixed-output part sets the feedback ratio internally; an adjustable part uses a supported external network.

In a low dropout regulator circuit, the input supply still needs enough voltage to support the output and the required headroom. The device also has a minimum operating input voltage. These requirements can differ, especially in architectures that use a separate bias supply.

An LDO does not use an inductor-based switching stage to regulate the rail. That can simplify the external circuit, but noise from the regulator itself and disturbances arriving from the input remain design considerations. The ABLIC introduction and TI's regulator seminar in the sources describe this feedback arrangement.

Dropout voltage changes with load and operating conditions

Dropout is the input-to-output voltage difference at which the regulator can no longer maintain the specified regulation condition. Manufacturers define and test it using stated criteria. Compare those conditions before comparing two headline numbers.

As an illustrative check, a 3.3 V output requiring 0.2 V headroom at the intended load needs at least 3.5 V at its input for that dropout condition. Supply tolerance, cable loss, ripple and temperature may require additional allowance. This estimate does not override a higher minimum input-voltage requirement in the datasheet.

Read worst-case dropout at the relevant current and temperature. A typical low-current value can be misleading for a heavily loaded rail. Operating near dropout can also change ripple rejection and transient behavior before the output becomes obviously unusable. TI's dropout guidance explains why headroom and the pass-device architecture must be considered together.

Calculate LDO heat and efficiency before selecting a package

For a first estimate, pass-device dissipation is approximately (VIN − VOUT) × IOUT. Ground-current consumption contributes additional input power. With an assumed 5 V input, 3.3 V output and 200 mA load, the first estimate is 0.34 W. At 12 V input with the same output and load, it rises to 1.74 W.

Ignoring ground current, efficiency is approximately VOUT/VIN: about 66% in the 5 V example and 27.5% in the 12 V example. These calculations describe assumed operating points, not measured product performance. The thermal resistance and board construction determine how much the junction temperature rises.

If a dc dc converter supplies the LDO, check the minimum instantaneous voltage at the LDO input. Nominal voltage minus nominal dropout is not enough when the upstream rail has tolerance and ripple. Thermal shutdown should be a protection event, not the normal method of managing dissipation.

PSRR, output noise and transient response describe different errors

PSRR describes how input ripple transfers to the output under stated test conditions. It varies with frequency, load, headroom and external components. Output noise describes noise generated by the regulator, typically integrated over a specified frequency band. A high PSRR number does not establish low internal noise.

Select a high psrr ldo against the disturbance frequencies that reach the input. A specification at a low frequency may not predict attenuation of a switching fundamental or its harmonics. Check the curves at a comparable load and input-to-output difference.

Load-transient response describes the temporary output change when current demand changes. The control loop, output capacitor and interconnect all contribute. A sensor's analog front end may need a quiet supply during acquisition, while a digital device may create abrupt current steps. Evaluate the real operating pattern rather than only a constant-current bench load.

Output capacitors, startup and reverse current need explicit checks

Use the capacitor range and ESR conditions specified for the exact regulator. Effective ceramic capacitance can fall with DC bias, so the printed capacitor value may overstate what the regulator sees. Extra output capacitance can also change startup time, inrush and discharge behavior.

Some regulators need a minimum load for their specified behavior; others do not. Noise-reduction, soft-start and feedforward capacitors have different functions and should follow the device guidance. A circuit copied from a related family is not necessarily stable with a replacement part.

Review what happens if the input collapses while the output remains powered by a capacitor or another source. Reverse-current blocking is a device feature that must be verified. Also check enable thresholds, power-good behavior and sequencing requirements where several rails interact.

Frequently asked questions about low-dropout regulators

Can an LDO raise the input voltage?

A conventional LDO regulates down from an input supply to a lower output. It cannot sustain an output above the available input by linear regulation. If the source can fall below the required rail, consider a suitable boost or buck-boost stage, with any LDO post-regulation chosen against the remaining headroom.

Is the lowest dropout voltage always the best choice?

No. Low dropout helps when headroom is limited, but noise, ground current, stability, accuracy and thermal performance may be more important elsewhere. Compare dropout at the intended operating current and temperature. A favorable typical number at a light load does not establish the best complete design.

Does an LDO remove all switching noise?

No. Ripple rejection depends on frequency and operating conditions, and layout can introduce paths around the regulator. Check PSRR at the relevant switching frequencies and headroom, then measure the assembled rail using an appropriate setup. The regulator also contributes its own output noise.

Why does my LDO become hot with a light-looking load?

The voltage difference matters as much as the load current. Even a moderate current can produce considerable dissipation when the input is far above the output. Calculate the loss, include ground current where relevant, and assess the board's thermal path against the manufacturer's limits and derating guidance.

Can I replace an LDO with one that has the same output voltage?

Only after reviewing pinout, input range, current, dropout, capacitor stability, enable logic and protection behavior. Reverse-current and startup characteristics may also differ. Output voltage alone does not establish compatibility, even when the package dimensions appear identical. Verify performance with the actual load and supply conditions.

Prepare an LDO component inquiry

Send ChipApex the input range, required output and tolerance, continuous and peak current, noise requirements, ambient temperature, capacitor constraints and package. Include an existing part number for replacements. These conditions make it possible to compare candidates against the rail rather than one dropout number.

References

• ABLIC: what an LDO is and how linear regulation works
• Texas Instruments: understanding low-dropout regulators
• Texas Instruments: LDO dropout fundamentals
• Texas Instruments: TPS735 datasheet, including stability and operating conditions