LDO peak-to-peak voltage 280mV - Page 1
Forum section: electronics / beginners

author: stafil
author_rank: Regular Contributor
timestamp: February 27, 2020, 07:59:40 am
content: I have an LD1117V33 connected to a 12V powersupply. When I plug the output to my osciloscope I see about 280mv peak-to-peak voltage, sometime more.Is it normal? Is there anything I can do to improve it?My circuit is pretty simple. 12V - LDO - Output with 2 capacitors in parallel in input and output. 100nF input and 10uF output.

author: mvs
author_rank: Frequent Contributor
timestamp: February 27, 2020, 12:25:55 pm
content: Quote from: stafil on February 27, 2020, 07:59:40 amTimebase 2µs/div? These noise peaks are quite fast... LD1117 has no chance to cope with them.It is normal, if you use switching PSU.Ferrite beads and additional ceramic caps may help you. The only question is, if you really need clean supply.

author: stafil
author_rank: Regular Contributor
timestamp: February 27, 2020, 08:45:05 pm
content: I've seen the noise go up to 500 pk to pk. I am afraid it will be out of the spec for my microcontroller.

author: wraper
author_rank: Unknown
timestamp: February 27, 2020, 09:14:22 pm
content: This probably does not even have anything to do with LDO but how you are doing measurement. Say long probe ground lead and noise induced from power source.Quote10uF of what type? If it's MLCC, LDO may oscillate.

author: stafil
author_rank: Regular Contributor
timestamp: March 03, 2020, 07:31:58 am
content: Yeah MLCC

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2020, 10:35:36 am
content: Quote from: wraper on February 27, 2020, 09:14:22 pmGoing by the data sheet, it should be stable, with a low ESR capacitor. It's not a very low drop-out regulator, because it has an NPN output device. I think the ripple is coming from whatever is connected to the input.https://www.mouser.co.uk/datasheet/2/389/ld1117-974075.pdfQuote from: stafil on February 27, 2020, 08:45:05 pmI doubt your microcontroller will be damaged, but it could cause problems with analogue circuitry.The ripple is probably coming from your 12V supply, rather than the linear regulator. You could try an RC filter in the output of the 12V supply. A 1000µF capacitor and 4R7 resistor will have a cut-off frequency of just under 34Hz and will greatly attenuate the ripple which is in the 1MHz region.EDIT:Make sure the resistor has a high enough power rating. P = I2R, so for the full 0.8A, the regulator is capable off I = 0.82*4.7 = 3W, but use a much higher rating. I'd recommend at least 8W, as the regulator might not current limit until 1.3A. You could also use three 1R8 3W resistors in series, with a 470µF capacitor to 0V, after each, to spread out the power dissipation and provide a steeper roll-off.

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: March 03, 2020, 10:39:36 am
content: I'm not sure 10 uF ceramics were so commonly used at the time the LD1117 was designed. The datasheet specifically calls for electrolytics ... see the app note in section 7.Try the improved ripple rejection version suggested in that app note, with a third capacitor ... and if that doesn't work, try a 1 Ohm resistor in series with the output capacitor.

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2020, 11:15:51 am
content: Quote from: I wanted a rude username on March 03, 2020, 10:39:36 amThat's a fair point. A ferrite bead in series with the capacitor might also help.Also try probing the 12V input to see if it's just as noisy or worse.

author: wraper
author_rank: Unknown
timestamp: March 05, 2020, 08:12:46 pm
content: Quote from: stafil on March 03, 2020, 07:31:58 amThen you either need to replace capacitor with different type, add series resistor to increase capacitor ESR or use different IC which is MLCC tolerant.

author: wraper
author_rank: Unknown
timestamp: March 05, 2020, 08:15:28 pm
content: Quote from: Zero999 on March 03, 2020, 11:15:51 amOne of worst fixes possible. Not only it negates capacitor effect at high frequencies it also does not increase ESR by known amount at lower frequencies. If you want to add anything, add series resistor.

author: wraper
author_rank: Unknown
timestamp: March 05, 2020, 08:18:00 pm
content: Quotehttp://www.ti.com/lit/ds/symlink/lm1117.pdf

author: james_s
author_rank: Super Contributor
timestamp: March 05, 2020, 08:56:01 pm
content: Remember also that a 10uF mlcc is going to give you considerably less than 10uF when you have DC across it. This characteristic is not as widely known as it should be and many of us have been bit by it.