Fixed versus Adjustable Output LDOs: Flexibility Versus Precision

Fixed versus Adjustable Output LDOs: Flexibility Versus Precision

ersaelectronics.com

LDO regulators come in two broad flavors: fixed-output parts, with the feedback divider integrated on the die and trimmed at manufacture, and adjustable parts, which expose the feedback node so the designer sets the output voltage with an external resistor divider. The choice looks like a small detail, but it affects output accuracy, noise, component count, board space, and the engineering effort required to get the rail right. This article compares the two approaches and explains when each is the better choice.

1. How Each One Works

A fixed-output LDO has an internal precision reference and an internal divider that is factory-trimmed to set a standard output such as 1.8 V, 2.5 V, 3.3 V, or 5 V. The user connects only the input and output capacitors. An adjustable LDO brings the feedback node out to a pin, and the designer picks two external resistors whose ratio sets the output according to the reference voltage, commonly around 0.6 to 1.2 V. The output voltage is then the reference multiplied by one plus the ratio of the upper to the lower divider resistor. The internal reference is the same in both cases; the difference is where the divider lives.

1. Output Accuracy

A fixed-output part is usually more accurate. The internal divider is trimmed at the factory together with the reference, so the total output tolerance, including reference accuracy, divider ratio tolerance, and temperature drift, can be held to a percent or better over temperature. An adjustable part inherits the reference accuracy and the internal offset, but the external divider resistors add their own tolerances and temperature coefficients. A pair of ordinary resistors with one percent tolerance and a hundred parts-per-million per degree drift can easily add two percent of output error, which is unacceptable in many precision supplies. Using high-grade resistors helps but adds cost and board space. For rails that feed precision ADC references or clocking circuits, the fixed part is usually the safer choice.

1. The Noise Penalty of the External Divider

The feedback node is a high-impedance point inside the control loop, and the external divider resistors act as a noise source and a noise antenna. Resistor thermal noise scales with resistance, and the divider injects that noise directly into the error amplifier. High-value dividers, chosen to keep standby current low, produce more noise and are also more susceptible to board leakage and pickup from nearby signals. Many adjustable parts include a bypass pin that allows a capacitor across the upper divider resistor to filter reference noise, but that capacitor also adds a pole that must be compensated. A fixed part, with its low internal divider, avoids most of this and generally achieves lower output noise, typically by a factor of two or more compared with the same silicon used in adjustable form.

1. Quiescent and Divider Current

The external divider draws a small current from the output continuously. To keep that current negligible, designers typically choose divider resistors in the range of tens to hundreds of kiloohms, which still drains a few microamperes from the rail. In battery-operated systems that are always on, this added current can matter at the margins, and it is one more thing to budget. The fixed part has no external divider current because its internal divider is sized by the manufacturer for the best tradeoff between accuracy and power.

1. Flexibility and Inventory

The adjustable part's strength is flexibility. One device can generate any output within its range simply by changing two resistors, which is valuable during prototyping, in multi-project systems, or when the output voltage may change late in design. From an inventory perspective, a single adjustable part can cover several rails that would otherwise require several fixed part numbers. For a production product with a stable bill of materials, that flexibility is worth less, and the simplicity and accuracy of the fixed part usually win.

1. Layout and Stability Considerations

The feedback trace on an adjustable LDO is a sensitive node. It must be kept short, away from switching traces, and connected directly at the load if remote sensing is used. Any stray capacitance on the feedback pin shifts the loop poles and can cause instability or excess noise. The fixed part hides this node inside the package, which makes layout easier and more forgiving. Designers who use adjustable parts must also verify loop stability with the chosen divider values and output capacitor, because the feedback network is now part of the compensation.

1. When to Choose Which

Choose a fixed part when the rail voltage is a standard value, accuracy and low noise matter, and the design is in production. Choose an adjustable part when the required voltage is non-standard, when the same part will be reused across multiple rails or products, when prototyping speed matters, or when remote sensing and tracking are needed. In audio, precision measurement, and clocking supplies, the fixed part is usually preferred; in general-purpose power trees and development boards, the adjustable part is often the pragmatic choice.

Summary

• Fixed-output LDOs integrate and trim the feedback divider on die, giving better accuracy and lower noise.
• Adjustable LDOs use an external resistor divider, trading some accuracy and noise for voltage flexibility.
• The external divider adds resistor tolerance, drift, thermal noise, and a small continuous drain current.
• Adjustable parts simplify inventory across multiple rails but demand more careful layout and stability work.
• The choice comes down to whether precision and simplicity matter more than voltage flexibility.