What Makes a Low-Noise Voltage Regulator?

Created: August 13, 2021
Updated: October 10, 2024

Your ICs need stable DC power to operate properly, and low noise is a standard requirement for any DC regulator. In some cases, you might worry about an extra couple dB’s of load regulation as long as noise can be filtered, but applications requiring ultra-precise timing and components with very narrow input voltage ranges need very stable DC voltage with low noise content. So how can you qualify the exact limits of low noise, and what types of regulators are best for these applications?

If you’re never looked through voltage regulator specifications, you’ll probably have to rely on marketing-speak to find a low-noise voltage regulator for your next system. However, the regulator itself is not the whole story, and there are other components and layout strategies needed to ensure a regulator provides stable low-noise DC output. Here’s what you need to know about different types of low-noise regulators and how to select the best component for your needs.

Low-Noise Voltage Regulator Fundamentals

A low-noise voltage regulator can technically be built from any topology; how you get noise low enough for practical use is another matter to be dealt with by the component or circuit designer. The regulator component you choose will have many important specifications, but the following three will tell you basically everything you need about a regulator’s handling of noise (both internally generated and external).

Power supply rejection ratio (PSRR). In short, this spec tells you how much conducted noise (in dB) from the upstream power supply is rejected by the regulator. The error amplifier in an LDO ensures high PSRR (similar to a switching converter), although they are inefficient if high step-down is required.
Output regulation. This can be measured in dB with respect to a nominal value or as a percentage. Truly low-noise regulators should have very low output regulation value; the best components will have regulation ratings below 1%. This specification might also be defined in terms of an RMS output as a fraction of the nominal voltage output.
Noise power density. This shows the amount of noise you’ll expect to see over an arbitrary frequency range. The relevant range can be taken from DC up to very high frequency (100 kHz to 1 MHz), or up to the switching frequency for a switching regulator.

All three quantities listed above are frequency-dependent. Note that the noise power density encompasses all sources of internally generated noise, including any internal switching noise. These three specifications need to be considered together when selecting a low-noise voltage regulator. For example, low output regulation and noise power density are meaningless if the input has 0 dB PSRR. Similarly, PSRR doesn’t matter if the output regulation value is very high.

Building a Regulator from Discrete Components

Other supporting circuitry on the output from a voltage regulator or within a regulator IC itself can help further reduce noise on the regulator output. For example, if you’re building your own linear regulator from discrete components, using a capacitance multiplier is a simple but powerful way to bring down noise; this topology is often used in audio or low-frequency RF regulators without the need for exotic components. Another example is an active regulator, which provides theoretically perfect noise reduction.

If you want to get even further noise reduction, you need additional filtering on the input/output, or you need to use a highly engineered integrated solution. You should also consider how filtration on the output and your switching frequency influence noise content in your regulator.

The Role of Switching Frequency and Filtration

If you’re looking for a switching converter, you’ll be adding some extra noise onto the output from the regulator. In essence, a switching regulator exchanges low frequency ripple from an AC source into high frequency noise, which is spread across the bandwidth of the PWM signal. This should illustrate the reason why many low-noise voltage regulator components are LDOs; they don’t have any switching noise. Take a look at this article to learn more about switching converter design and how to choose the best frequency for your converter.

Example noise spectrum produced by a buck converter driven in two modes compared with CISPR limits. [Source]

Finally, if you need very low noise content from your regulator, a higher order low-pass filter can be used to remove some noise content from the device. These filters can be designed from passives, although be careful with your layout when working at high current with a switching regulator; the switching action generates a high magnetic field that can induce a current on the filter output which then propagates downstream to other components.

Two Example Low-Noise Voltage Regulators

STMicroelectronics, LD56100

For low-voltage, moderate current applications, the LD56100 series from STMicroelectronics is a low-noise LDO providing up to 5.5 V output with 1 A current. It provides very low dropout voltage of 120 mV with output stabilized using an integrated ceramic capacitor. PSRR is rated at 70 dB (1 kHz frequency) with 0.13 mV RMS noise at the 3 V output.

Block diagram for the LD56100 low-noise LDO regulator. Source: LD56100 datasheet.

Analog Devices LTC7151S

One example of a low-cost, low-noise voltage regulator is the LTC7151S from Analog Devices. This SMD low-noise regulator provides user-controlled adjustable switching frequency from 400 kHz to 3 MHz with an integrated PWM driver, making it an excellent choice for systems that need a high power switching converter with low loss. Input range spans up to 20 V, and output spans up to 5.5 V with 15 A delivered to a load. Two supply regulation modes are offered, both of which maintain output load regulation below 0.25% over the entire output current range.

LTC7151S low-noise voltage regulator application circuit. Source: LTC7151S datasheet.

Other Components for Power Systems

A low-noise voltage regulator doesn’t get down to low noise on its own, and your regulator section will require other components to provide the desired power output and noise floor. For very precise regulation in high power systems, maintaining stability and ensuring low noise simultaneously is a major challenge, requiring a range of other power management components. Here are some other components you might need for your product:

When you’re looking for a low-noise voltage regulator and other components for your new system, you can find all the components you need with the advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to current distributor pricing data, parts inventory, and parts specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our PMIC components page to find the components you need.

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