How to Determine Output Capacitance for an LDO?  - Page 1
Forum section: electronics / projects

author: kml183
author_rank: Contributor
timestamp: October 24, 2023, 07:11:24 pm
content: Hi there!I wondered how to determine the max capacitance a AP2112K-3.3 (https://www.diodes.com/assets/Datasheets/AP2112.pdf) can drive from the datasheet. The datasheet specifies that it is stable with 1u caps on the input/output, is that the max?Thanks!

author: wraper
author_rank: Unknown
timestamp: October 24, 2023, 07:25:46 pm
content: They state minimum capacitance, not maximum. Not to say what causes stability issues in many LDO with ceramic capacitors is not too large capacitance but too low ESR

author: AnalogTodd
author_rank: Regular Contributor
timestamp: October 24, 2023, 08:21:37 pm
content: A well designed LDO should have no maximum capacitance for the input and output, as a user may find that the transient performance when using a small capacitor is unacceptable and the best way to improve it is by increasing the capacitors. Tradeoffs with larger capacitors include longer startup times and slower overall response but with less deviation from the DC regulated output.With that said, there are circuits I have seen before where going beyond a given amount of capacitance kills stability and it is not something the manufacturer talks about.

author: magic
author_rank: Super Contributor
timestamp: October 24, 2023, 11:04:04 pm
content: I suspect that "too much capacitance" is most likely to cause problems if it also happens to mean "too low ESR".So definitely check specs on ESR and if there is a minimum, avoid paralleling enough capacitors to go below it.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2023, 11:12:27 pm
content: The datasheet only says that it works with 1 microfarad ceramic, aluminum electrolytic, and tantalum electrolytic, and ceramic will include film. For electrolytic capacitors I am sure it will work with any larger size, but it might have problems with larger ceramic or film capacitors. The only way to know is to ask them or test it.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 24, 2023, 11:21:14 pm
content: Of course ultimately you'll never know until you test the part in your particular use case. But as a general point, most modern linear regulators are designed to accomodate input and output ceramic capacitors just fine, and these already have very low ESR, even if you don't parallel several of them. So the low ESR problem is most often not one. This hasn't always been the case, of course, with older designs. The reason for that is quite obvious, market. If you try and sell LDOs these days that are not stable with just ceramic caps, except possibly with niche devices, you're not going to sell much.

author: magic
author_rank: Super Contributor
timestamp: October 25, 2023, 05:47:14 am
content: I would be curious if this means they are stable with purely capacitive reactance, or only with the ESR and ESL of "typical" ceramic caps and PCB layouts.I.e. could you make them oscillate by being sufficiently creative with extreme bypassing

author: David Hess
author_rank: Super Contributor
timestamp: October 25, 2023, 06:36:43 am
content: Quote from: SiliconWizard on October 24, 2023, 11:21:14 pmThe design solution is also not trivial because the missing ESR has to be moved to be in series with the power pass element. This results in either raising the dropout voltage or special construction of the power pass transistor which will not be available in all processes.

author: wraper
author_rank: Unknown
timestamp: October 25, 2023, 08:41:24 am
content: Quote from: SiliconWizard on October 24, 2023, 11:21:14 pm1117 are still quite popular and often are incorrectly used with MLCC on their output. But yes, it's not the best part to advertise when you release new LDO that is not stable with MLCC.

author: peter-h
author_rank: Super Contributor
timestamp: October 25, 2023, 11:15:29 am
content: Take a look at the TL431 data sheet, for examples of unstable regions, where more C can make it unstable but a lot more C makes it stable again. Also these vary according to the package!

author: Siwastaja
author_rank: Super Contributor
timestamp: October 25, 2023, 03:05:57 pm
content: Many regulators come with stability chart which explicitly shows amount of C and ESR and whether the regulator is stable with the combination. Also Vin can be a factor. Such specification does not exist for this part, so you have to assume/guess/test. It probably is stable with larger Cout as they advertise it's stable with MLCC, but it might not be.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 25, 2023, 03:08:03 pm
content: Quote from: wraper on October 25, 2023, 08:41:24 amYes, and many other non-MLCC stable or marginally "maybe-MLCC-stable" regulators, too. One should never assume low-ESR stability of a linear regulator based on the idea that "most" or "modern" regulators are stable.