LDO OUTPUT COMPONENTS - Page 1
Forum section: electronics / beginners

author: Nikos A.
author_rank: Regular Contributor
timestamp: November 14, 2019, 01:36:05 pm
content: Hi everyone,I was looking some designs and I have some questions regarding some components at the output of the LDOs.What is the purpose of the inductor used in the circuit below?What is the LG1 rated at 1,5A component?

author: ArthurDent
author_rank: Super Contributor
timestamp: November 14, 2019, 01:58:15 pm
content: LG1 is just a ferrite bead on a lead so its value is quite low. Both of the circuits you show look like they were designed for a SMPS where there would be lots of switching noise. If you Google the spec sheets for this regulator they only show one capacitor on the input and one on the output. Neither circuit looks bad but probably won't add any benefit in a battery powered circuit.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: November 14, 2019, 02:11:09 pm
content: Some LDOs are conditionally stable with output capacitors. Which means that they can oscillate with very high or very low value capacitors . One of the ways to solve the oscillation is to add a series resistance with the cap. Usually the ESR of tantalum caps serve this purpose. But they do leak a lot. If you use ceramic capacitors which have very less ESR, then it is necessary to add a series resistance to prevent oscillations. The schematic designer chose to add a ferrite bead/inductor to serve as the series resistance. It may offer some additional noise filtering but the main purpose here is to improve the stability of the LDO.

author: ArthurDent
author_rank: Super Contributor
timestamp: November 14, 2019, 02:26:58 pm
content: As far as stability of the regulator goes and using series resistors or not, I'll go with the information provided by the maker on the spec sheet:"Output voltages available for the MCP1702 range from 1.2V to 5.0V. The LDO output is stable when using only 1 μF of output capacitance. Ceramic, tantalum or aluminum electrolytic capacitors can all be used for input and output."

author: Nikos A.
author_rank: Regular Contributor
timestamp: November 14, 2019, 02:36:02 pm
content: Thank you guys!! What is the purpose of the inductor at the first circuit? In general, is it a good practice to include LC filters at the output of LDOs for noise suppression?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 14, 2019, 02:56:11 pm
content: The series inductor seems to be inviting disaster. Inductive loads on regulators can cause them to oscillate. You'll never see that in the datasheet, only the capacitive quadrant is specified -- if they give a region of stability at all, which they often don't. (An extreme example is the intentional use of a LM317 -- a notably stable regulator otherwise -- as a Hartley oscillator for medium wave radio. It's been done!)A ferrite bead or choke, between caps (a pi filter), is fine, and yes, can be used to help reduce white noise from the regulator, and switching noise coupled from the source. Better to put the filter in front of the regulator for the latter case, and better to use a regulator with reference filter pin for the former case.Tim

author: wraper
author_rank: Unknown
timestamp: November 14, 2019, 03:42:21 pm
content: Quote from: Nikos A. on November 14, 2019, 02:36:02 pmShowing that who designed that thing did not know what he/she was doing.QuoteNo, especially between LDO and output cap. Afterwards it's fine but you may have some voltage drop on it. I would consider using ferrite beads on output only If I have multiple things to power from single LDO and I want to prevent noise transferred between them. If you are concerned about filtering noise from power source, it's way better to place it before input capacitor of LDO.In case of second circuit, output capacitor is not even properly selected. MCP1702 requires 1uF minimum directly on output.

author: Nikos A.
author_rank: Regular Contributor
timestamp: December 10, 2019, 09:09:44 am
content: Thank you guys!!I have also found one application note and an article regarding ferrite beads that both were really helpfulhttps://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdfhttps://www.analog.com/en/analog-dialogue/articles/ferrite-beads-demystified.html

author: xani
author_rank: Frequent Contributor
timestamp: December 10, 2019, 06:54:30 pm
content: When in doubt, look into datasheet first rather than copying some random circuit as the components used might be there not for reasons you think they are there.For example, for the first one the LT datasheet says "Stable with 3.3μF Output Capacitor" (on datasheet there is single tantalum there) and for MCP1702 it is "Stable with 1.0 μF to 22 μF Output Capacitor" and datasheet's example circuit just have 1uF ceramic at input and output.None of them mention additional coils/ferrite beads. The guess of it having something to do with filtering high frequency ripple is probably correct but the place where they are put are weird at best. Might be the case of "I slapped the indoctor/bead there and it started working, good enough"

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2019, 08:18:30 pm
content: The safe solution if LC filtering is required is for the output capacitor required for stability to be attached directly at the regulator's output with the LC section following it as shown in the second example. It is usually more effective to place the following LC section at the load.The inductor should be lossy to limit Q and often includes a low value parallel resistor for this reason. In old designs you may find the inductor wound around the resistor. Ferrite beads for this application are designed to be lossy.The current rating on the inductor or ferrite bead is to avoid saturation which will lower the inductance and effectiveness of the filter.

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2019, 10:23:26 am
content: If the inductor had a damping resistor in parallel, it would make more sense, since at high frequencies the regulator would see the capacitor, with the series resistance, which would be bypassed, by the low resistance inductor at low frequencies, thus avoiding the associated voltage drop.A ferrite bead is a good idea because it behaves as an inductor at low frequencies and a resistor at high frequencies, where the regulator is likely to oscillate.