How Do I filter out This 52 kHz fixed frequency internal oscillator on a LM257

sam_sam_sam

Dose any one have any idea how i should filter out this noise

I want to use this in a Project using a Basic Stamp

I have use this switching regulator before with a Basic Stamp and LCD Display but there was only a 120 millamp
on the switching regulator

This did not cause any problem for me but will loading down the regulator to more Than

·One amp or more through this switching regulator will this cause me any problem

Is this 52 kHz· noise that Switching Regulator is making while under a load ·going to make the Basic Stamp·reset it self

I took some reading with my meter and there is 52 kHz· noise· when it is under load

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
··Thanks for any··that you may have and all of your time finding them

·
·
·
·
Sam

PJAllen

Might depend on how much "noise."

Maybe it's your construction technique?· Did you use ground-plane or single-point technique with short-lengths·as they advise and emphasize or are the ground wires a bunch of point-to-point and so on?· That's very important.· How about a picture of it?·

sam_sam_sam

PJ

single-point technique with short-lengths·as they advise and emphasize

Thank for your reply

I tied all the ground together as much as possible and used one very short wire between the two caps

all part the leads are as short as possible

Unless I just have to have a board made to do this project

I will try to take a photo of the board tomorrow

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
··Thanks for any··that you may have and all of your time finding them

·
·
·
·
Sam

Peter Jakacki

There will always be some switching noise due to the on/off nature of the regulator and the cyclic nature of the feedback sensing (ramp-up, switch off, ramp-down, switch on). With the 2575 this should be around 20mv which is not a problem in digital systems. Normally I use a higher frequency switcher which means that I can use smaller inductors and caps plus the higher frequency ripple can be filtered out with a post filter if necessary. Easiest way is simply to use a slightly higher voltage and feed that into an LDO regulator.

*Peter*

P.S. I have attached a diagram of the switching waveform.

Post Edited (Peter Jakacki) : 1/4/2009 3:41:16 AM GMT

sam_sam_sam

PJ

Maybe it's your construction technique?· Did you use ground-plane

Thank You for point this out that was what it was it

I had to make a soilder brige to all the grounds together

and that fix the problem

This was the frist time that I made my own switching regulator board

I have seen this before using a ground plane but I did not understand why you need to
use one

Now I understand why·····

Thanks agian· ·

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
··Thanks for any··that you may have and all of your time finding them

·
·
·
·
Sam

sam_sam_sam

Peter Jakacki

·I use a higher frequency switcher

Thank You for your reply

Do you have a part # of one that you use and a web link to·the Data Sheet

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
··Thanks for any··that you may have and all of your time finding them

·
·
·
·
Sam

Peter Jakacki

Instead of the LM2575 I would use the LM2675 in an 8-pin SMD or DIP package. The part is rated for 1A just like the LM2575 but it only needs a 47uH inductor and a 68uFcapacitor as it operates at 260kHz. This part is also footprint compatible with the LM2594 which runs at 150kHz and only handles 0.5A but this is the one that I use in most of my products.

Layout is simple but I always have a ground plane. Make sure though that the ground current flows through the power-supply section if you know what I mean. The switching regulator must be the source for the regulated supply and ground, not just a stop along the way or worse still, the destination.

That is, don't have the input ground passing through all your circuitry first and then to the regulator but arrange the pcb so that the regulator is first inline with the input ground, layout is everything. (I think I said that right)

*Peter*

sam_sam_sam

Peter Jakacki

Do you have a lay out that·I could look at LM2675 that you have used before and that works

Thank you for all of your help in this matter

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
··Thanks for any··that you may have and all of your time finding them

·
·
·
·
Sam