LM5118: Noise on FB pin from FET Gate.

FB Pin images.

5684.Arina 24V DC-DC 4Amp.PDF

Hi John,

thank you for using the E2E forum.

Let me look into this, but due to a ongoing team training this might take a few days.

I should be able to comment on this in the next 1-2 days.

Best regards,

 Stefan

OK. I have images of the noise at the FB pin but am having issues upload.  

I have removed the OPAMP drive into the FB to focus on the noise.

FB Pin with full 2A load at 24V.

FB pin with no load at 24V.

Got it.  Special charter in the name.....

Hi John,

i am not sure if I now put all information you provide together in the right way.

So you have removed the OPAMP which is also connected to the FB pin - right.

And you see the signal shown above (i guess you are talking about channel 2) on the FB pin and wondering about the high spike you see there.

Not sure what the first picture should show me?

Can you add a picture of your measurement setup - esp. the connection of the Probe to the FB signal.

Best regards,

 Stefan

Yes the OPAMP is removed by removing R11 and R25. 

The first image is the Q3 FET, which is the half of the design that I am using to connect to the load.

I measure the noise on the exposed pad after removing R11 and R25.

I will work on a measurement picture and try and get a scope capture with the Gate and Noise at the same time.

Hi John,

thanks, just to ensure, it would be great to also see the measure ent setup snd the probe connection to the board.

Best regards,

Stefan

I used a shorter GND on the scope probe which did reduce the noise seen.

Still at about 100nS and bid swing over the 1.23VDC FB.

R22 FB Expanded Image

R22 FB Noise

Gate Signal

Probe setup

Hi John,

as you already have seen the loop of the measurement probe does pick up a lot of noise.

Additionally i recommend to enable the 20MHz filtering on this input of the probe, in my opinion this is still some switching noise pick up from the probe you see here on the scope.

Best regards,

 Stefan

Well it looks like I never had a problem.  With the bandwidth reduced it look very clean.