Oscillating regulators with ceramic caps

Chris Jones · 2015-03-28T15:45:34+00:00

I've just built up a batch of PCBs with several LM317 and LM337 regulators on them. As this thing will be in a hot environment, I wanted to avoid electrolytics so I used 10uF MLCC ceramic capacitors on the inputs and outputs of the regulators. Now that I have built it, all of the regulators oscillate! I had thought that it was only LDOs (with the drain or collector of the pass device connected to the output) that would oscillate with low-ESR capacitors, but I now know that this also applies to some non-LDO linear regulators. Having seen the oscillation, I looked in the LM337 datasheet and sure enough it says it isn't stable with low-ESR capacitors. It does not even say what range of ESR it IS stable with. In the case of LDOs, I already had the policy of refusing to design in the ones that require some ESR without specifying what range is acceptable. By experiment, for the LM337LM I have found that somewhere in the range of 50mOhms to 100mOhms added between the regulator and 10uF MLCC seems to just barely stop it from oscillating, without any other decoupling capacitors connected. I would use a somewhat higher value to get more phase margin. Adding these resistors is a pain because I have a lot of other MLCCs decoupling the same rails, so there is no nice place to hack in the resistors, especially since the board was not routed with this in mind. If I put the resistor between the regulator and the power plane, then there will be some interaction between the different circuit blocks as they would then share a significant common supply resistance at DC. Perhaps an inductor? Meh. I guess there is a good reason why ADI can charge more for their AnyCap LDOs. I wonder if any of them is pinout compatible with the surface mount LM317MDCYRG3 and LM337LM that I have used. Chris

Mar 28, 2015 24 Replies

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John Larkin 11 years ago

The ESR of wet aluminum electrolytics skyrockets below 0C. Using them is a good way to build a temperature-activated oscillator.

Polymer alums are good over temperature, but their ESR may be too low for some regulators. Tantalums are just right for 1117 type regs.

John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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Tim Williams 11 years ago

They'll be better off than the regs -- 100C-ish cutoff, and that's Tj on top of whatever power they're dissipating, so I hope this thing isn't actually needing an ampere of capacity and/or an ambient over 60C.

Tim

Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com

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M.Randelzhofer 11 years ago

like that low ohmic resistor R for debugging and measuring current too, but as i understand the R is after the cap, so the mlcc problem for LDOs still exists.

I like to put a current measure R between the cap and the feedback voltage divider, so the V drop is regulated out. Disconnecting the load for debugging is done by putting the feedback divider on another pad connected directly on the cap output, additionally put on the layout.

MIKE

www.oho-elektronik.de OHO-Elektronik Michael Randelzhofer FPGA und CPLD Mini Module

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Phil Hobbs 11 years ago

No, the resistor goes between the regulator and the cap, but the voltage divider is on the regulator side. Otherwise the output voltage would go nuts.

Cheers

Phil Hobbs

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David Eather 11 years ago

That does sound an awful lot like what happens with LDO's - there is a range of output cap that will oscillate. Never heard of it for a 317 though. A re-spin or an LDO in drag?

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