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Original question:

LM22673: Soft start pin damage reason

LM22673: Soft start pin damage

Martin Eibl
Prodigy 50 points
Part Number: LM22673
Other Parts Discussed in Thread: LM22672

I am using the LM22673. I use the part twice on one board, for 3.3V and for 5V output with an input voltage of 24V. Over the last 3 years I got more than 10 defect PCBs back from customer. Sometimes 3,3V is defect sometimes 5V is defect. Its always the Softstart pin (100nF),  "damaged", means a short to ground (several Ohms up to 1k).  Any recommendations to solve the issue?

see schematic R201 (56k) is not populated

  • +1
    TI__Mastermind 35210 points

    Hello,

    Sorry to hear about your issue.

    If you meant to include any attachments I cannot see them.

    Could you please provide the schematic and layout for review?

  • 0 in reply to Martin Eibl
    TI__Mastermind 35210 points

    Thank you for re-attaching this info. I can see it now.

    Below is my review.

    Schematic

    • What is the intention of R201? Pin5 SS typical application is to have cap to ground for programming soft-start time
    • Current limit is set for 4A peak. Ensure L203’s power rating can support it
      • Same goes for V201. I cannot make off the p/n
    • An un-damped 2nd stage filter (L206) can be problematic for device stability. Please review this appnote for how un-damped 2nd stage filter’s can impact stability and how you should damp them for proper DC/DC design.

    Layout

    • C201,C209 is far away from the input pin. I advise locating it closer if possible.
  • 0 in reply to Marshall_Beck
    Prodigy 50 points

    Many thanks for the review

    R201 is not populated (done by BOM)

    current limit is set bei 18k (Iadj) should give 2A limit according the data sheet, or is my assumption not correct?

    V201 is a MBRS3200, schottky with 3A rating

    L203 rating is 4.5A, saturation current is slightly more

    L206 is a ferrit beat I have to check Bode plot

    Any ideas why the SS Pin is affected / defect?

    regards

    Martin

  • 0 in reply to Martin Eibl
    TI__Mastermind 35210 points

    Hi Martin,

    You are welcome.

    Martin Eibl said:
    current limit is set bei 18k (Iadj) should give 2A limit according the data sheet, or is my assumption not correct?

    Yes that seems about right.

    Martin Eibl said:

    Any ideas why the SS Pin is affected / defect?

    That I cannot be positive to. The device is becoming overstressed one way or another. Perhaps it could be handling/assembly ESD or something in your system that can be inducing damage on it. Maybe something attached to Vout which has a path to FB which has a path to SS.

    Martin Eibl said:

    R201 is not populated (done by BOM)

    So R201 is never used in any designs? What is its intention of being on the board? 

  • 0 in reply to Marshall_Beck
    Prodigy 50 points

    Hi, R201 was never used in that design, its from a former design with LM22672 and builds a voltage divider for the EN Pin, instead of C205 a resistor to GND was placed.

  • 0 in reply to Martin Eibl
    TI__Mastermind 24115 points

    Hi Martin,

    Marshall is on vacation and will get back to you once he returns. 

    Thank you for your patience,

    Joshua Austria

  • 0 in reply to Joshua Austria
    TI__Mastermind 35210 points

    Hi Martin,

    I would suggest you preform some level of accelerated lifetime testing on your design in a controlled environment.

    This could be evaluating your board at high temperature in a temperature chamber at full operating  conditions.

    If you are able to replicate a failure, then, this would expose a systematic issue with the DCDC design.

    If not, then, there may be a system level or additional issue occurring in the assembly/handling of the device, like ESD damage.