question on LM3477

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raphaelriv

New Member

• Dec 29, 2006

• #1

i have a couple of these built that are giving me an unstable output voltage that varies by 0.1V ... when i take out the capacitor from pin 4 to pin 8 it seems to solve the problem except the output drops by 0.3V... my schematic is the one shown in the pdf page 1 as the 'typical high efficiency step-down converter' .... i lack the understanding of this, any ideas?


http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/4/LM3477.shtml

 

Hero999

Banned

• Dec 29, 2006

• #2

What do you mean it varies by 0.3V?

That capacitor should always be there for stability's sake.

What sort of PCB layout are you using?

It's often critical for switching regulators like this.

 

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raphaelriv

New Member

• Dec 29, 2006

• #3

re:

i mean the voltage output when 12V provided is suppost to be 7.2V its 7.11-7.22-7.33 then back again to 7.11

here's my schematic

 

audioguru

Well-Known Member

Most Helpful Member

• Dec 29, 2006

• #4

The schematic is bigger than my house!
It is more than half a mega-byte.
I wonder why.

 

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raphaelriv

New Member

• Dec 29, 2006

• #5

re:

no reason ..

 

dknguyen

Well-Known Member

Most Helpful Member

• Dec 29, 2006

• #6

I can't take in all of that schematic, even if I tried.

 

audioguru

Well-Known Member

Most Helpful Member

• Dec 29, 2006

• #7

I tried making the schematic smaller but then the writing couldn't be seen.

 

Roff

Well-Known Member

• Dec 30, 2006

• #8

I saved it, opened it with Windows Picture and FAX Viewer, rotated it 90 degrees, and it's perfectly readable. You can zoom and maintain resolution. Unfortunately, I still don't know why the circuit doesn't work as expected.

 

Hero999

Banned

• Dec 30, 2006

• #9

Poor/incorrect PCB layout?

Faulty componants?

Assembly errors, dry joints, bridging?

 

Roff

Well-Known Member

• Dec 30, 2006

• #10

If you are running the part lightly loaded, it will switch to hysteretic mode, with high output ripple.

 

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raphaelriv

New Member

• Dec 30, 2006

• #11

re:

what do you mean loaded?

 

audioguru

Well-Known Member

Most Helpful Member

• Dec 30, 2006

• #12

Loaded. Very, very drunk.

 

mvs sarma

Well-Known Member

• Dec 30, 2006

• #13

Hero999 said:

Poor/incorrect PCB layout?

Faulty componants?

Assembly errors, dry joints, bridging?

Click to expand...

Most probably lay out and current paths. Great HERO999 !!!

 

Hero999

Banned

• Dec 30, 2006

• #14

raphaelriv said:

what do you mean loaded?

Click to expand...

He means without anything connected to the output.

I'm not meaning to be nasty but is English not your first language? Perhaps you should fill your location in on your profile.