BUCK switching waveform ... problem

[x1215]

I have a 5V to 1V bulk which is built in PMIC, the schematics diagram is as below picture

I measure at the point LX, the waveform as below is wierd, I think it should be square waveform, but it is a little distored.

Need help what is going on?

 

[Bluejets]

What is it like on the other side of the inductor?

 

[x1215]

@Bluejets
Output voltage waveform is not good, due to sink current change fast.

 

[Bluejets]

So am I reading this correctly?

You have a 26mv spike lasting 2.5usec?
What is the load?

 

[x1215]

So am I reading this correctly?

You have a 26mv spike lasting 2.5usec?
What is the load?

YES you are right.
Load is CPU's V-CORE, current consumption MAX to 2A.
Because I am not much familiar with DCDC buck, so I don't know if the LX signal is fine or not.

 

[duke37]

If Q1 turns on fully then the LX waveform should have a flat top at 5V.

I thought that Q2 should have a diode across it to take the current while Q2 is turning on.

 

[WHONOES]

If Q1 turns on fully then the LX waveform should have a flat top at 5V.

I thought that Q2 should have a diode across it to take the current while Q2 is turning on.

No, the waveforms shown are quite normal.
The circuit shown is a Synchronous Rectifier type where the normal diode is replaced with Q2. Q2 would normally be on when Q1 is off.

The output ripple can be caused by a number of things.

• Parasitic capacitance of the inductor windings. This allows the fast edges of the switching waveform to partially bypass L.
• Poor ESR of Cout. Large electrolytic types will exhibit this. Several smaller value types will help to mitigate it. A few 100n ceramic caps will also help.
• Poor layout will also contribute. One of the main candidates here would be poor grounding. All the high current points should sum at one junction and not be daisy chained.
• A lot of the rubbish on the output could be removed by the use of a high impedance Cylindrical Ferrite core on the output leads.
  

 

[BobK]

Is a load connected?

Bob

 

[WHONOES]

Loading the output will make the ripple worse.

 

[BobK]

Some switchers do not work well with no load.

Bob

 

[WHONOES]

I agree but, that generally affects their ability to regulate properly.

 

[x1215]

the output have a ARM V-CORE sink，I will try to put some 100nF first.