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TPS62080: Output Voltage above set voltage

Freek Rogge
Prodigy 130 points
Part Number: TPS62080

We use this design on out board to create the internal voltage of the FPGA, it is set to 1.2V

On a few board we get about 1.7V output and the regulator isn't switching, it only switches at startup until it reaches 1.2V, the regulator stops switching and the output keeps rising until 1.7V

The first capture is zoomed in on the first part of the second capture

 The yellow line is Vout, the blue one is Vswitch (pin7)

VCC is 3.3V

There are over 100 boards where it works perfect, but there are e few boards where we have this same problem.

What could be the cause of this failure?

  • 0
    TI__Expert 3645 points
    Hello,
    To support you, can you provide me also the layout of your application?
  • 0 in reply to Alfonso Furio
    Prodigy 130 points

    Hello,

    Red is the top layer, blue is bottom layer, the layers in between are Ground planes

    Vint_FPGA goes via the bottom layer to the FPGA on Top layer.

    The capacitors C113, C141 and C136 are on Bottom layer under the FPGA.

    The problem looks similar to this case https://e2e.ti.com/support/power-management/f/196/t/595647?tisearch=e2e-sitesearch&keymatch=TPS62080

    But all the capacitors are ceramic not tantal and I don’t have a ferrite bead in series.

    The circuit works on over 200 boards but on a few boards we have this problem.

    I think it is an assembly error, but I can't find the error.

  • 0 in reply to Freek Rogge
    TI__Expert 3645 points
    Hello,
    At the moment I don' t notice anything strange in your schematic and layout. Did you check if the boards with this problem have all the pads adequately soldered? You can try to replace the feedback divider of one board to see if there is an assembly problem at that point. Also, If you did previous tests (with related conditions) on these devices, please let me know.
  • 0 in reply to Alfonso Furio
    TI__Intellectual 2605 points
    Hello Freek,

    Do you have any feedback regarding Alfonso's suggestions?

    Thanks and best regards,
    Yann
  • 0 in reply to Yann Ducommun
    Prodigy 130 points

    Hello,

    sorry for the late reply, I was on vacation.

    The divider network seems ok, The soldering also seems ok. Only the bottom pad I can not check.

    I have ordered new parts to replace, I will let you know when I replaced the parts.

  • 0 in reply to Freek Rogge
    TI__Expert 3645 points
    Hello,
    To not have a post opened for a long time, I will close it. If you need further support, you can open a new thread, and I will be glad to support you.
  • 0 in reply to Alfonso Furio
    Prodigy 130 points

    Hello,

    Sorry for the late reply. I was waiting for new parts.

    But I've replaced the part by a new one, I got the same result, about 1.7V.

    I also checked the working boards and I saw that all the boards produce 1.7V at start-up, after 50ms the voltage drops to 1.2V.

     

    Yellow is Vout, blue is measured at Pin7 SW.

    Do these devices need a minimum load to work properly? 50ms is approximately the FPGA start-up time.

    The boards where we have noticed this problem are unprogrammed boards and so they are not running.

    What can cause this 1.7V?

    Kind regards,

    Freek Rogge

    Atos Engineering

     

  • 0 in reply to Freek Rogge
    TI__Expert 3645 points
    Hello,
    Can you tell me how many boards have this behavior? Last time you said to me that this problem wasn't noticed on around 100 boards and now I didn't understand if also those boards show the same voltage drop at the output. If possible, I would also have the scope plots of the inductor current and PG like in figure 28 and 29 (page 18 of the datasheet). Which load are you using and is it also the same of the previous post?
  • 0 in reply to Alfonso Furio
    Prodigy 130 points

    Hello,

    - I had a few boards that didn't work and I was checking all the voltages and on these boards I noticed that Vint_FPGA is about 1.7V

      On A working board the Vint_FPGA is 1.2V as it should be.

    - But than I was checking the start-up on a working board and all our boards (about 300) act the same, the start-up is like my post from yesterday.

    - When the boards are booting or didn't start the Vint_FPGA is 1.7V and drops to 1.2V the moment the FPGA is started.

    I will try to measure Inductor current. 

    The load is the internal core of a Spartan6 FPGA.

  • 0 in reply to Freek Rogge
    Prodigy 130 points

    Hello,

    It seems there is something else in the circuit that causes this 1.7V at start-up.

    When de regulator is disconnected from the FPGA it is 1.2V from the beginning.

    I have to find the "leak", it looks there is no problem with the regulator.