TPS54292

Can you post your schematic and some waveforms that illustrate your issue?  Are there any differences in teh layout between the output that is acting up and normal outputs?

I will do so.  Let me go collect what you need.

Thanks,

-j

The top shot shows the switching signal.  The pulse width varies here  from 350ns to narrower 260ns.  The lower shot shows the 50kHz signal at the output.

6646.TPS54292.pdf

It looks like that output is unstable.  Switcherpro suggest that your compensation is chosen correctly.  It may be your load.  What is that 4 V driving?  Is there a lot of distributed capacitance?  Have you tried shorting the ferrite bead?

The only thing being driven by the output is an +3.3V LDO.  There is a 10uF cap at the input of the LDO and it is about 12 inches away.

I did remove the two 22uF caps at the output on either side of the ferrite bead and replace them with a single 10uF cap on the switcher side of the bead.

When I remove the ferrite bead so it's not driving anything, it does become stable.

Here's what the layout looks like.

Thanks,

-j

I was suggesting that you replace the bead with a wire shunt.  I also have heard of cases where LDO inrush current causes problems with dc/dc converters that are driving them.  Can you tell me which LDO you are using?

I did replace the bead with a wire shunt, no change.  I'm using an LT1764AEQ-3.3 LDO.

http://cds.linear.com/docs/en/datasheet/1764afb.pdf

Thanks,

-j

If you remove the bead to disconnect the LDO, can you load the 4 V out put with a resistive load?  That would verify whether the LDO is causing the problem.

Hey John,

I'll give it a try tomorrow morning when I'm back at the equipment.

Thanks,

-j

Hey John,

What I did:

Removed bead and loaded switcher with a 1A draw.  No oscillations, looked clean.

With bead installed I removed the 10uF Cap at input to LDO.  Oscillations remained.

With bead removed I lifted pins 1 and 2 on LDO and ran a wire externally between switcher and LDO.  This was a fail.  I had bursts of switching like the switcher was limiting and shutting down.  The frequency of the bursts were in the 110Hz range.

This does look like interaction between the switcher and LDO.  As I mentioned before,d I'm using 3 of these with 6 outputs.  Three of the 6 drive LDOs 5.5V -> +5VA, 4.0V->+3.3VA and 2.5V -?> 1.8VA

After closer look, I am also getting a 40kHz component on the 2.5V output, but it is small; not booming like the 3.3V.  The 5.5V output is clean.  All three are supplying LT1764AEQ LDOs.  The 1.8V and 3.3V LDOs are fixed and the 5.5V LDO is adjustable.

What I found:  The fixed LDOs have a sense pin.  I ran the sense pin to after the bead to account for any drop through the bead.  Bad Idea.  To fix, I lifted the sense pin and tacked it to the Vout pin and all is clean.

Thanks,

-j