Non-LDO linear regulator stability with ceramic bulk capacitors - Page 1
Forum section: electronics / projects

author: daqq
author_rank: Super Contributor
timestamp: December 16, 2015, 11:39:03 am
content: Hi guys,I'm going to be using a linear regulator (LM317/LM337 or 78L09/79L09) for a project. There is a lot of head room between the input voltage (+/-12V) and the output voltage (+/-9V). I know that LDO devices have stability issues when working with big, low ESR capacitors. Does the same apply to non-LDO devices?I've looked through a few datasheets and application notes - they do not contain any explicit ESR requirements, nor any recommendations against ceramic caps. How can I determine whether the device will be stable with, say, 4.7uF ceramic?Thanks,David

author: mariush
author_rank: Super Contributor
timestamp: December 16, 2015, 11:49:55 am
content: Some of them do not like very low esr, they expect at least around 0.1 ohm esr. It's not mentioned because most datasheets have 4.7uF - 10uF electrolytic or tantalum capacitors shown in the examples and such capacitors by default have higher than 0.1 ohm esr.Some datasheets are also quite old and if you're lucky just revised from time to time, so when they say ceramic capacitors you have to think what ceramic capacitors were available in - let's say - 1990-2000 and what impedance those capacitors had, compared to modern ceramic capacitors. I would say with LM317 or 7809 it would be safe to use ceramic capacitors, but with others.. be careful.For example, see 1117 regulators (from various manufacturers), it's a ldo that doesn't like very low esr at the output and it says so in the datasheet :http://www.ti.com/lit/ds/symlink/lm1117-n.pdf(page 9)

author: langwadt
author_rank: Super Contributor
timestamp: December 16, 2015, 12:25:14 pm
content: Quote from: daqq on December 16, 2015, 11:39:03 amthe 337 uses and NPN output and is negative it is the same configuration as an LDO and has the same stability issues

author: tszaboo
author_rank: Super Contributor
timestamp: December 16, 2015, 01:52:00 pm
content: If you must use ceramic, just place a sub 1 ohm resistor in series with the capacitor. There is no benefit with the low ESR anyway, you need that close to the load.

author: daqq
author_rank: Super Contributor
timestamp: December 16, 2015, 02:44:01 pm
content: Thanks guys. The main reason I want to use them is because we use them in large quantities already.The small series resistor seems the best solution, since the current draw will be around 30mA anyway...

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2015, 05:12:48 pm
content: The series resistor is just in series to the capacitor, not to the load. Often it is better to have not just 1 capacitor, but a combination of a low ESR and one with some ESR. Depending on the regulator, the cap does not need to be large, so something like 100 nF is usually enough for a LM317 or similar. The improvement with an additional electrolytic 10 µF capacitor is more due to being a capacitor with some ESR than just more capacitance.Additional low ESR capacitors are better placed at the load.

author: nctnico
author_rank: Super Contributor
timestamp: December 16, 2015, 05:49:18 pm
content: Quote from: NANDBlog on December 16, 2015, 01:52:00 pmIn my experience placing a large ceramic further away from the LDO solves the oscillation issues IF you are not using power planes.