Satisfying min esr requirement for ncp 1117adj? - Page 1
Forum section: electronics / beginners

author: bobbydazzler
author_rank: Regular Contributor
timestamp: July 15, 2021, 08:11:10 am
content: I want to use the 1117adj with ceramic input capacitors which according to the datasheet need an esr between 0.2-20ohms, I was wondering if having some resistors in series with the input pin(not in series with the capacitor) would satisfy these requirements?I think the charging of the ceramic input capacitors would happen at an esr of whatever my resistors are but the discharging of the capacitors would probably have an esr of <0.1ohms. Would this still be OK?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 15, 2021, 08:33:58 am
content: The capacitor at the input side of an LDO usually has no need for a minimum ESR. For the input side lowest ESR is usually best, if there are no other reasons, like another regulator or a possible resonance in the cable. Against a cable resonance a series resistor can help, but is still limited and a combination with a ferrite may be better. This is not related to the regulator.The ESR is usually critical for the output side. Here the resistor should be in series to the capacitor. It can be still ok to have another small cap in parallel with low ESR - e.g. at some parts to supply. Isolating some circuit parts with series resistors can also contribute to the ESR needs. The requitement for the regulator is usually not directly the ESR, but avoiding a pure (low loss) capacitive load impedance in a certain frequency range (e.g. 10 kHz to 10 MHz). A single cap with ESR is ok for this. However a real cicuit often has more parts, like a cap at the regulator and additional caps at each chip.The RC combination is most effective in damping at 1/(2pi RC).

author: Siwastaja
author_rank: Super Contributor
timestamp: July 15, 2021, 09:13:25 am
content: You posted an image of 78xx datasheet even though you talk about 1117.Input capacitor can have arbitrarily low ESR, although I suggest damping the input with a high-ESR capacitor as well, especially if the input comes from long wires.On the output, a ceramic cap with series resistor does the trick indeed, but having two parts sucks IMHO. Also remember the minimum capacitance requirement of the 1117, and remember worst offender MLCC capacitors easily drop their capacitance by 80% under DC bias.Tantalum is a typical choice on the output. Derate voltage by 50%., i.e, a 20V part is good to 10V. Do not use tantalum in the input because they are sensitive to unlimited surge currents.Finally, I would seriously consider not using 1117. It's ancient, sold as LDO but by modern standards not very low-drop at all, have that stupid output capacitor requirement which increase the cost and area of the complete solution, even if 1117 itself is cheap.Look at distributor parametric search and see which regulators you can get today; look for availability, good price, suitable footprint for you, then check the datasheet, modern parts often brag on the front page being stable with ceramic output cap, and such small values as just 1µF, with such low drop-outs like just 300-400mV. These modern parts are better in every regard and easier to use.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: July 15, 2021, 09:32:25 am
content: I just posted the image to show the config of my input capacitor in relation to the series input resistors, I have 16v tantalum's for the adjust and output but they can't cope with the 12v input without exploding hence why I needed to use ceramic.Sounds like low ESR on the input is a non concern so I'll just use the ceramics there then.