LM5005MH rectangle signal at pin switching node

Maik,

I am going to move this post out of the signal switches and multiplexers forum and into the non-isolated DC/DC to see if they can support your request.

Thank you,

Adam

Here you see the schematic and a waveform of the sw-signal at 15 volt input voltage. With an external voltage regulator we connect 8 volt as mentioned in the datasheet.

Hello,

Its difficult to say from the scope shot what is happening.  We can start by ruling out Sub Harmonic Oscillation.  Please can you further reduce Cramp to 220pF and see if the Switch node Stabilizes?  Failing that can you provide a pdf of the layout.  In particular interested in the position of the Cin capacitor as it relates to the IC. 

Also please can you also provide a BOM of the components you have used?

Thanks,

Hello,

i changed Cramp to 220pF and the signal at the switching node is stable until an input voltage of 37 volt. I had to change the Cramp to 100pF to get a stable signal from 10 to 60 volts (L1 is 33µH). Iout is about 1.2A so the ti webtool says L1 = 68µH, Cramp 680pF, but if i change the components to the suggested i have the distortions again.

Hello,

The issue is to do with subharmonic oscilation, you will need adequate ramp to stabilize.  Please go ahead and reduce Cramp until you obtain stable operation over the vin range.  Hope this helps?

David.

Hello, 

I changed the frequency of the LM5005Mh to 260 kHz and Cramp to 100pF and now it looks good over the whole voltage Vin. Thanks for your help.

Maik