LDO regulators

A low-dropout regulator is usually the last active device between a noisy or loosely regulated rail and a circuit that needs a steady voltage. At Haiben we supply the Weforsemi line of power ICs alongside the wider catalogue, and this page explains how to specify an ldo regulator from that range: what the dropout figure means, where noise and PSRR decide the part, and which parameters purchasing and hardware engineering should agree on before a quotation.

What an LDO regulator is chosen for

An ldo regulator is a linear regulator operated with a small input-to-output differential. Inside, a pass transistor is driven by an error amplifier against a reference, so the output is held at a fixed voltage while the excess input voltage is dissipated as heat. That simplicity is the reason LDOs remain on most boards: no switching node, no inductor, few external parts, and a quiet output.

The trade-off is efficiency. Because the pass transistor drops the difference between input and output, power loss rises with that difference and with load current. An LDO makes sense when the headroom is small, the load is modest, or the rail must stay clean. When the gap is large or the current is high, a switching converter does the step-down and the LDO cleans up afterwards. Our page on LDO vs buck converter sets out that division in more detail.

One boundary worth stating once: this page covers low-dropout parts. Three-terminal regulators in the classic 78-series style, with their larger dropout and fixed pinouts, sit on the sibling linear voltage regulators page. The two families are related but specified differently.

Dropout: the number that frames the whole choice

Dropout voltage is the smallest input-to-output differential at which the regulator can still hold its output in regulation at a given load current. Below that differential the pass transistor saturates and the output follows the input down. For a buyer, dropout is the first filter: it determines the minimum input voltage the rail can survive, and therefore how much margin remains when a supply sags or a battery discharges.

Two practical points follow. First, dropout is quoted at a specified load current and rises with it, so the figure that matters is the one at your maximum load, not the headline number. Second, dropout and quiescent current pull against each other in many architectures; a part optimised for one is rarely optimal for the other. Define both before comparing options.

Working through a specification means fixing, at minimum:

• Input voltage range, including the worst-case low point during transients
• Output voltage and tolerance, fixed or adjustable
• Maximum and typical load current
• Acceptable dropout at that maximum current
• Quiescent current, particularly for always-on rails
• Package and thermal path, since dissipated power is (VIN − VOUT) × IOUT

The thermal item is often the deciding one. A small package with a large input-output difference and a real load current will heat up regardless of how good the electrical specification looks.

Noise, PSRR and the clean-rail jobs

The reason to choose an LDO over a switching converter is frequently noise. Two figures describe it. Output noise is the regulator's own contribution, quoted over a bandwidth and often as a spectral density. Power-supply rejection ratio (PSRR) describes how much of an input ripple is attenuated at the output, and it falls as frequency rises, so it must be read at the frequencies your upstream converter actually produces.

Typical clean-rail jobs include analogue front ends, sensor signal chains, reference and bias circuits, audio paths, and clock or RF sections where supply ripple turns into phase noise. In each case, the LDO is fed by a pre-regulated rail and its task is to remove what remains. PSRR, output noise and the noise contribution of the reference are the selection criteria; dropout is usually easy to satisfy because the upstream rail is already close.

Layout and external components affect the result as much as the part. Input and output capacitors, their ESR, and the placement of the ground return all shape stability and rejection. The datasheet's recommended capacitor ranges are part of the specification, not an afterthought.

Specifying an LDO from this range

Our Weforsemi line covers charging ICs, LDOs, DC-DC boost ICs, DC-DC buck ICs, backlight driver ICs and LED driver ICs. Low-dropout regulators appear in the official catalogue as the Low-Dropout Regulators (LDO) category, one part of the broader power management ICs offering that also includes buck and boost converters, LED drivers, three-terminal regulators, overvoltage protection ICs and lithium battery charging ICs.

To specify an ldo regulator from this range, send us the list from the dropout section: input range, output voltage and tolerance, load current, dropout limit at that current, quiescent current target, package preference, and any noise or PSRR requirement with the frequency band that matters. Exact part numbers, datasheet values and current availability are confirmed at quotation rather than published here, so the response you receive is tied to the specific part we are offering.

Purchasing staff can treat the same list as the basis for comparison between quotations: same rail, same load, same thermal assumptions. Where a design sits between an LDO and a switcher, we will say so rather than force the part.

Frequently asked questions

Can Haiben supply an LDO regulator with a specific fixed output voltage?

Yes, subject to the range available at the time. Give us the required output voltage, tolerance, input range and load current, and we will confirm which Weforsemi LDO options match. Exact part numbers and datasheet values are confirmed at quotation.

How do I choose between an LDO regulator and a buck converter?

Compare the input-to-output difference against the load current. Small difference and modest current favour an LDO for simplicity and low noise; a large difference or high current favours a buck converter, often followed by an LDO where a clean rail is needed. Our LDO vs buck converter article covers the trade-offs.

Are the three-terminal 78-series regulators on this page?

No. Those parts are covered on the linear voltage regulators page. This page deals with low-dropout regulators, which are specified primarily by dropout at load current, quiescent current, noise and PSRR.