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TPS40200 smokes micro

VBKen
Intellectual 255 points
Other Parts Discussed in Thread: TPS40200, TPS54062

I have designed a circuit using your TPS40200 to convert 28V to 5V.  Without anything hooked up, I get a good 5V output.  When I connect a 5V micro and power the the circuit from a 28V battery, the micro gets smoked (literally) every time.  I suspect the instantaneous power available from the battery couple with power-on transients on the 5V output are blowing the micro.

I have seen several suggestions:

Increase the soft-start delay - is there a limit to the amount of delay?  0.047uF SS cap blow the micro.  0.1uF SS cap is a bit better, more like a slow blow.  Any suggestions?

Add a snubber to the output

Add a TVS diode to the output

Does anyone have any idea as to why the micro is blowing and any suggestions on how to fix the problem?

Thanks,

  • 0
    TI__Mastermind 35165 points

    Hi Ken,

    I have a few questions based on what you have given. My best guess sounds the same as yours, there is some overshoot at the micro due to a transient condition.

    1. Can you post a schematic of your design?
    2. How close is the output of the TPS40200 supply to the input of the micro on the layout? A long distance increases parasitic inductance and can cause voltage overshoot.
    3. Have you looked at the output voltage during the turn on transient directly at the micro? There may be some overshoot causing the 5V micro to fail. A larger SS cap will help for overshoot during startup. One thing to be aware of with larger SS capacitors is a longer period for the restart in an overcurrent condition.
    4. What type of output current transients do you expect for the TPS40200 supply? There might be overshoot on the output due to load current being removed.

    Regards,
    Anthony

  • 0 in reply to Anthony F
    Intellectual 255 points

    Anthony,

    The schematic is based on the design produced by SwitchPro.

    The input voltage is 28V.  The output cap C2 is 33 uF.

    The micro is located about 2.5" from the output of the switcher.  The micro requires less than 10 mA.  The component values are overkill for the design but were chosen from those available for high temperature operation.

    The documentation from switchpro states that the soft start cap C5 should not be changed.  I would like to try a 1 uF cap.  A 0.1uF cap only slightly improved things.

    Any suggestions?

    Thanks

  • 0 in reply to VBKen
    Intellectual 255 points

    Here is the schematic:

  • 0 in reply to VBKen
    TI__Mastermind 35165 points

    Thank you Ken.

    Can you get some screenshots of the output voltage and switching node (node between FET, Inductor, and diode) when it starts up? If you can do this with and without the micro connected it would be good. But I understand not wanting to blow up another one on purpose.

    Off the top of my head I do not see an issue with a 1uF slow start capacitor if the application can live with the longer over current protection hiccup period and start up. I think it would be good to confirm the voltage is overshooting causing it to fail first.

    Also for 10mA load current, is there any reason you cannot use something like the TPS54062? To me this could be a much simpler solution.

    Regards,
    Anthony

  • 0 in reply to Anthony F
    Intellectual 255 points

    Anthony,

    After designing, building and testing the initial prototype board, I sent spare boards and parts to my client for assembly.  It turns out that one of the resistors on the newly assembled board was shorted resulting in a high voltage on the output.  The micro didn't stand a chance.  After repairing the board, the problem went away and the micro powered up correctly so, problem solved!

    As far as the choice of switchers is concerned, the TPS40200 was chosen because it is also available in a high temperature version.

    Thanks