question on LM3477

[raphaelriv]

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]

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.

 

[raphaelriv]

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]

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

 

[raphaelriv]

re:

no reason ..

 

[dknguyen]

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

 

[audioguru]

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

 

[Roff]

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]

Poor/incorrect PCB layout?

Faulty componants?

Assembly errors, dry joints, bridging?

 

[Roff]

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

 

[raphaelriv]

re:

what do you mean loaded?

 

[audioguru]

Loaded. Very, very drunk.

 

[mvs sarma]

Poor/incorrect PCB layout?

Faulty componants?

Assembly errors, dry joints, bridging?

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

 

[Hero999]

what do you mean loaded?

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.