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TPS65251 unstable output

Piotr Błachnio
Prodigy 40 points

Hello,

Yesterday I launched my Power supply for FPGA and unfortunately all 3 outputs were unstable. 

This is my first experience with DC-DC converter and I have no idea what to do, what may be wrong?

If you have any suggestion I would be glad to hear it. 

Here you have my shcematic (it's typical schematic from datasheet, only resistors to set the proper output voltage were changed )

PCB layout also was rewritten from datasheet 

And this is how my output of 2,5V and 5V looks like:

I will accept any help :)

Regards 

Piotrek

  • 0
    TI__Guru**** 191445 points
    Can I see the SW node waveforms as well as SS?
  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    Also can you check the load current? You may be tripping the over current and going into hiccup mode.
  • 0 in reply to JohnTucker
    Prodigy 40 points

    Thanks for answer, but after some measurements something has changed, first in 5V output than after some time in 2,5V output.

    Output is dead until turning off the converter - while turning off then output looks like this (5V) just before complete shut down.

    While power is turned on voltage on LX3 pin looks like this:

    Frequency looks almost like the switching frequency (508Khz calculated for my resistor) but why 11,6V?

    This repeats all time, I zoomed to better see the shape.

    About the current:

    To 5V output I connected red led + 330ohm resistor so I may "see" this ripples (previously) and the pick when turning of the device. To 2,5V I also connected led but with smaller resistor about 60-70 ohm.

  • 0 in reply to Piotr Błachnio
    TI__Guru**** 191445 points
    That is how a switching converter works. You should see a pulse train at the SW node at your operating frequency. The pulse amplitude will be just about Vin. The average value = Vout. From your previous waveforms, it looks like teh device is shuttting down and re-starting. It could be hiccup current limit. if the OC last mor than 10 msec, it can shut down all three converters.
  • 0 in reply to JohnTucker
    Prodigy 40 points

    Yes, it looks like you are right, it is hiccup effect. But how can I reach current limit with just led + small resistor at output?

    R47 sets current limit to 3A.