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TPS54319: Bad Startup, Oscillating?

Part Number: TPS54319

Hi All,

I have a series of boards with the TPS54319 stepping down 5V to 2.5V. When functional they pull less than an amp of power. For some of my boards, they start up just fine, but for some they start oscillating at 35.7kHz and never come up above 100mV.

I've isolated these converters from the rest of my circuit and they still won't come up, leading me to believe these are the problem. I've also attached a 100Ohm resistor from their output to ground in case they need a load to startup. Still no dice. 

Not sure if this is a red herring, but if I place 2.5V on their output rail with an external PSU, then take away the support, the converters are able to operate stably afterwards. Seems to be an issue with startup? 

I'm not sure what to believe, since in all cases these chips were dead on arrival. The schematic is also shown below.

Any thoughts on what might be causing this? 

  • Sorry. The labels for these scope grabs are Yellow is 5VIN. In the top photo both blues are attached to a different channel's 2.5V test point on a single functional board. In the bottom scope grab, purple and dark blue are attached to the same 2.5V test points on a board exhibiting this bad behavior. That fuzz is the 35kHz oscillation.
  • Hi,

    Where does the 5V come from? It seems not very stable, and the input capacitor is 10uF/16V/0603, the real value is 5uF with 5V bias voltage.
    The output voltage waveform is very strange, please double check it. Please disconnect the last stage and next stage circuit, and try to power the board with external 5V power supply, to check if the issue still occurs.

    In addition, could you please share the layout? Let's check if there is any issue.