LM5148-Q1: Clickling sound from switch mode circuit

Hello Thomas,

Looks like some instability, does not appear normal, can you share your schematic and also be kind enough to fill out design calculator spreadsheet with your requirements so I can check your design against it.

4251.LM5148-LM25148 Quickstart Calculator.xlsm

Thanks.

Hello David,

Thank you for the answer.
I have already used the spreadsheet which I have attached, together with a schematic.

1614.LM5148-LM25148 Quickstart Calculator_rev3.xlsm

Thank you

Hi David,

Thank you for the suggestions.

1. Tried to short out this extra 2uH inductor (L202), but that did not change anything.

2. Used 50% of the populated capacitance in the spreadsheet and changed to compensating components, but no change either.

But if you say it looks like instability then I guess I just have to play around with the compensating components.

Another thing to add is that it only makes this click sound when I switch the load ON/OFF. Duing the ON or OFF periode there is no sound.

Having done some more measurememts, I think I maybe had a bad connection when measuring the SW node. Now it looks more stable, still changing the duty-cycle. I have attached some new pictures of the SW node. 

Both with no load.

Thank you.

Hello David,

Try to do some more measurements, which I have listed below together with some measurements details.

This is where a load is switched ON/OFF and the yellow trace is the output AC coupled.

Two plots of SW node, with no load and with persistance ON. They look and behave pretty much the same with or without load.

Output ripple AC coupled with no load.

SW node rise/fall time under full load of some 14.5A

All in all I think the switch waveform looks fast and sharp. I have an external 5V supply to VCCX.

Another point to mention is that the jitter seems lees when I activate 2 channels on my scope, which could indicate something about the sampling as it goes down with the number of channels. My scope is RTB2004 300MHz, 2.5GSa/s which should be plenty fast.

I hope you can conclude a little more on these plots, if there is an issue?

Thank you very much and if anything else is needed please let me know.

Br. Thomas

Hello David,

As soon as you mentioned the FPWM, I came to think about the CNFG pin and that I have a 42.1K ohm resistor. This enables the spread spectrum feature. I have now changed this resistor to 27K and the jitter is as good as gone.

Br. Thomas

Hi David,

Still some jitter on the back slope, but maybe that is normal. I will maybe experiment with the compensation network if this can improve it.

Yes was also thinking this clicking noise is from the inductors.

Br. Thomas

The responsible engineer is out of office now. The response will be delayed. 

-EL

Hi David,

Yes have now tried to change the crossover frequency and thereby the compensation network, but not really changed anything.

I will try with your suggestion and play with the capacitors.

Thank you for support.