Output Noise in LDO Regulators: Sources and Low-Noise Design

Output Noise in LDO Regulators: Sources and Low-Noise Design

ersaelectronics.com

The whole reason to choose an LDO over a switching converter is often that its output is quiet, but quiet is not zero. Every LDO produces a small amount of electrical noise on its output, and that noise sets the noise floor of everything downstream, from audio amplifiers to RF synthesizers and precision analog-to-digital converters. Understanding where this noise comes from, how it is specified, and how external components shape it is essential when the rail feeds a noise-sensitive block. This article walks through the origins of LDO output noise and the practical techniques used to keep it low.

1. The Main Noise Sources

The output noise of an LDO originates almost entirely inside the integrated circuit rather than from the load or the board. The dominant contributors are:

• The voltage reference. The bandgap reference that sets the regulated level generates inherent thermal and flicker noise, and this noise is amplified and passed to the output.
• The error amplifier. Its input transistors and resistors contribute thermal and 1/f noise, which the loop then drives through the pass element.
• The feedback network. The divider resistors add thermal noise, scaled by the divider ratio.
• The pass element itself contributes relatively little voltage noise, because its noise is largely rejected by the loop, but its on-resistance can add small thermal terms.

Because these sources are uncorrelated, they combine as a sum of squares rather than a simple addition, and the reference is usually the largest single contributor.

1. How Noise Is Specified

Datasheets report output noise in two complementary ways. The first is integrated output noise, expressed as a root-mean-square voltage over a defined bandwidth, commonly 10 Hz to 100 kHz or 0.1 Hz to 10 MHz. This single number is useful for comparing parts and for checking against a total noise budget. The second is noise spectral density, plotted in microvolts per square-root-Hertz against frequency. The spectral density curve shows the shape of the noise: a rising edge at low frequency indicates flicker, or 1/f, noise, while a flat region indicates white thermal noise. The shape matters because a circuit sensitive to audio bands cares about a different part of the curve than one sensitive to RF.

1. Spectral Density: What the Curve Shows

At very low frequencies, the noise density is high and falls as frequency rises, following the 1/f characteristic of semiconductors. Above a corner frequency, often a few kilohertz, the noise flattens into a white, thermal floor. Typical low-noise LDOs have an integrated output noise in the range of a few microvolts rms over the audio band, while general-purpose parts may be ten times higher. The spectral density curve also reveals how the noise-reduction pin changes the picture: adding capacitance there pushes the 1/f corner down sharply, which is why low-noise parts are usually used with that pin populated.

1. The Effect of External Capacitors

External capacitors do not merely stabilize the loop; they also filter noise. The output capacitor forms a low-pass impedance with the output resistance of the regulator, attenuating noise above a corner frequency set by the capacitor value. A larger output capacitor therefore reduces high-frequency output noise, although it cannot remove noise below that corner because the loop already regulates there. The input capacitor has little effect on output noise directly, but it prevents supply noise from reaching the pass element and indirectly helps PSRR. The noise-reduction capacitor, connected from the NR or bypass pin to ground, filters the reference and is the single most effective external component for lowering noise in the audio band.

1. Bandwidth and Noise Tradeoffs

There is a fundamental tension between low noise and fast response. A wider loop bandwidth lets the regulator correct disturbances more quickly, but it also passes more of the internal noise to the output. Conversely, filtering the reference heavily to reduce noise narrows the bandwidth and can slow the loop. This is why very-low-noise LDOs sometimes have softer transient response, and why selecting a part requires matching the noise target to the speed target. Designers should not assume that the quietest part is always the right one for a fast digital load.

1. Low-Noise Design Practice

To achieve the lowest practical noise:

• Choose an LDO rated for low output noise, with a spectral-density curve that is already flat and low in the band of interest.
• Always populate the noise-reduction pin with the recommended capacitor, typically on the order of 10 nF to 0.1 uF.
• Use a generous, low-ESR output ceramic capacitor to add high-frequency filtering.
• Keep headroom moderate; excess dropout does not reduce noise, it only adds heat.
• Isolate the sensitive analog rail on its own LDO rather than sharing a noisy digital rail.
• Lay out the capacitors close to the pins and use a quiet ground return, because board coupling can undo the best silicon.

Summary

• LDO output noise comes mainly from the reference, the error amplifier, and the feedback resistors.
• It is specified both as an integrated rms value and as a spectral-density curve, and the curve reveals the 1/f and white-noise regions.
• The noise-reduction pin capacitor is the most effective external tool for lowering noise in the audio band.
• Output capacitance attenuates noise above the loop corner, while the loop bandwidth sets a tradeoff between noise and speed.
• Low-noise design combines a suitable part, the right bypassing, adequate headroom, and careful grounding.