how to remove negetive spike? - Page 1
Forum section: electronics / projects

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 05:42:53 am
content: hii have a tps54331 and build 4V DC with it.in digital scope when i see single shot of it , it has negative spike around -250 mv during start up.i have a output filter like this in output in attachmenthow can i remove negative spike?

author: rs20
author_rank: Super Contributor
timestamp: March 06, 2016, 05:47:40 am
content: A schottky diode in parallel with the capacitors?

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 05:52:04 am
content: Quote from: rs20 on March 06, 2016, 05:47:40 amill test it but my negativespike is -250 mv . is there any schottky that work with -250mv?

author: rs20
author_rank: Super Contributor
timestamp: March 06, 2016, 05:53:57 am
content: You're correct, a schottky is unlikely to turn on a 250mV. Why do you care about a -250mV spike? No IC will be damaged by that.

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 05:55:00 am
content: Quote from: rs20 on March 06, 2016, 05:53:57 amits for RF Device that its important to them.

author: Phoenix
author_rank: Frequent Contributor
timestamp: March 06, 2016, 05:56:43 am
content: Can you upload a picture from the oscilloscope and show where on the circuit you are probing?

author: rs20
author_rank: Super Contributor
timestamp: March 06, 2016, 05:57:33 am
content: Quote from: navidrct on March 06, 2016, 05:55:00 amYou need to give more detail. Is the spike occuring once during start up, or is it correlated to the switching frequency of the TPS54331?And what specifically are you worried the -250mV spike will do? Damage your device, or just introduce noise?

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 05:59:31 am
content: Quote from: Phoenix on March 06, 2016, 05:56:43 amactually no because its in RF department and i don't have any access to it . but i probing the output of filter

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 06:00:56 am
content: Quote from: rs20 on March 06, 2016, 05:57:33 amits during start up . and im worried about damaging devices.

author: rs20
author_rank: Super Contributor
timestamp: March 06, 2016, 06:07:07 am
content: Quote from: navidrct on March 06, 2016, 05:59:31 amWhere's your ground lead? Are you sure you're picking up a real signal?Quote from: navidrct on March 06, 2016, 06:00:56 amI'm not really qualified, but I call [citation needed] on the claim that devices will get damaged, even fancy RF voodoo. Damage occurs due to current flowing through parasitic diodes, overheating them, and creating a short circuit that renders the device inoperable. If the voltage isn't high enough to even turn a Schottky diode on, that sequence of steps is averted at the first step?But assuming your concerns are legitimate, it is odd that you're getting a negative spike. Is it purely negative, or ringing that goes both positive and negative?

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 06:12:31 am
content: Quote from: rs20 on March 06, 2016, 06:07:07 amyes im sure in probing.its ringing in positive as well

author: rs20
author_rank: Super Contributor
timestamp: March 06, 2016, 06:16:16 am
content: Well, I suppose your LC filter is ringing, then? Is it at the right frequency for that to be the explanation? What frequency is the ringing at?Perhaps you could use diodes in parallel with the inductors to clamp any ringing.

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 06:23:02 am
content: Quote from: rs20 on March 06, 2016, 06:16:16 amits during start up and take 120 us to settlei don't know if diode work because its just -250 mv

author: rs20
author_rank: Super Contributor
timestamp: March 06, 2016, 06:24:57 am
content: Diodesin parallel with the inductors. If the DC-DC is outputting 4V, and the output after the filter is -250mV, that's 4.25V across the series pair of diodes in that configuration, clearly more than enough to turn them on.And what is the frequency of the ringing, not the time it takes to settle?

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 06:33:28 am
content: Quote from: rs20 on March 06, 2016, 06:24:57 amspike is in start up time and is one time thing and its freq will be 1/120us

author: navidrct
author_rank: Regular Contributor
timestamp: March 06, 2016, 06:57:18 am
content: Quote from: blueskull on March 06, 2016, 06:55:09 amThat is my opinion too but they didn't accepted itSent from my HTC Desire 820ts using Tapatalk