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WEBENCH® Tools/TPS5450-Q1: unstable output as voltage rises

Jeffrey Drost
Prodigy 20 points
Part Number: TPS5450-Q1
Other Parts Discussed in Thread: TPS5420, TPS5450

Tool/software: WEBENCH® Design Tools

Hello,

We've been using the TPS5420 successfully in various designs but need to move to the TPS5450.  We are having trouble getting stable voltage from the unit with the parts we have chosen.  We went back to Webbench and bought its recommended parts for our parameters (24v in) and we still have the same issue, when the input rises above 7.5v , the output starts to rise as well.  From 5.5 to 7.5v the output is stable at the desired voltage of 3.3v, then for every 1 volt input increase , the output goes up about 0.5v.  Which component would most likely affect this?  I tried a couple different inductor values but the situation remained.

Jeff

  • 0
    TI__Guru**** 191445 points
    What is ENA connected to? Is it pulled up to VIN? Can you post your actual schematic as well as some waveforms that demonstrate your issue? Be sure to include the PH node.
  • 0 in reply to JohnTucker
    Prodigy 20 points

    John,

    Thank you for the response.  The ENA is floating. The schematic shows our first attempt with components we selected, then we went back and populated the Webbench BOM and got similar results.  Our layout is very close to the datasheet I think, we have used the TPS5420 with a similar layout and hd no issues.  The wave forms are with the Webbench components.  The first waveform is at 6.0v Vin, channel 1 is the 3.3v after the inductor, channel 2 is PH.  The second waveform is when I raise the Vin to 7.4v.

  • 0 in reply to Jeffrey Drost
    TI__Guru**** 191445 points
    It appears that the TPS5450 has stopped switching. Since there is no ac current in the inductor, both ends are at the same voltage. Try 10 uf ceramic capacitor directly across VIN and GND pins.