Author Topic: Lc filter after regulator?  (Read 4144 times)

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Offline XaeroDemonTopic starter

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Lc filter after regulator?
« on: July 20, 2016, 08:10:38 am »
Can somebody proof the schematics please....Is this even correct ?? I am not sure.I know people use lC filter before Regulator(??)but can u use a lc filter after a regulator? IS THERE A reason not to do it? thanks!!!
 

Offline SeanB

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Re: Lc filter after regulator?
« Reply #1 on: July 20, 2016, 08:30:59 am »
You need a capacitor as well directly on the regulator output, otherwise you will have an oscillator. The LDO regulator is not stable without an output capacitor, the datasheet tells you a recommended value and also min/max limits on ESR of the capacitor as well. Otherwise it will work and reduce noise slightly.
 

Offline TrickyNekro

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Re: Lc filter after regulator?
« Reply #2 on: July 20, 2016, 08:38:44 am »
Best way is to go with common mode choke if it´s a must.

You should put a schottky between the regulator and the inductor and from the other side connected
to the ground and pointing to the first connection.

You should use dampening, rule of thumb, parallel to your existing capacitor you connect a snubber circuit,
snubber capacitor 10 x the filter cap and a resistor equal to the minimum impedance of the filter at resonance.

You will be much better off to just use a regulator with less noise and / or depending on what you need better CMRR figure.
If not go for a RC, which yeah I know it´s going to be big on the PCB but, LC is really overkill.

Regards,
Lefteris
If you are an engineer and you are not tired...
You are doing it wrong!
 

Offline KE5FX

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Re: Lc filter after regulator?
« Reply #3 on: July 20, 2016, 10:56:40 am »
... and the amplitude of said ringing may be quite a bit higher than you (or your load) expect. 

Probably the easiest way to improve the situation is to add a low-value series resistor -- maybe 5 ohms or so, depending on the current and acceptable voltage drop.  But there goes your load regulation. 
 

Offline T3sl4co1l

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Re: Lc filter after regulator?
« Reply #4 on: July 20, 2016, 05:08:26 pm »
Good way to make an oscillator:
http://www.qsl.net/qrp/tx/317-tx.htm

Tim
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Bringing a project to life?  Send me a message!
 

Offline XaeroDemonTopic starter

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Re: Lc filter after regulator?
« Reply #5 on: July 21, 2016, 06:30:49 am »
Thanks Every body for replying ... I am Not gonna go RC bcus voltage drop is not acceptable for and the load.I am using a ULN reg(20uVrms 10hz-100khz) for 3.3V in series so I guess I don't "Need" a LC filter.But the source is pretty noisy so I am gonna use a LC filter anyway.@TrickyNekro I cannot use a snubber,schottky  bcus the board space is kinda limited |O.Thanks again Guys 
 

Offline VK5RC

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Re: Lc filter after regulator?
« Reply #6 on: July 21, 2016, 08:21:34 am »
For a laugh one of my radio mates got a LM317 up to 5MHz.
Whoah! Watch where that landed we might need it later.
 


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