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LMZ31530: High impedance observed between PGND and AGND of LMZ31530

Shenbagavalli S
Prodigy 20 points
Part Number: LMZ31530

Tool/software:

Dear Team,

We are using LMZ31530RLGT for 0.95V generation.It is working fine for past three years.Now 0.95V is not generated, while debugging comes to know that high impedance observed between PGND and AGND of LMZ31530.As per data sheet PGND and AGND is shorted internally and no need to short externally.

We are having provision to short AGND and PGND in our PCB design. When we are using this resistor to short PGND and AGND, output voltage is generated properly.

Is it advisable use this external resistor to short AGND and PGND? Please provide your suggestions or valuable feed back as soon as possible.

Regards,

Shenbagavalli S

Data Patterns (India) Ltd,

Chennai.

  • 0
    TI__Expert 8446 points

    Hello Shenbagavalli,

    While the AGND and PGND external connection is not recommended, it does provide a workaround and the device would generally operate as expected (there are some conditions where GND noise may cause unexpected behavior related to sensitive pins connected to AGND). What is the failure rate here? How many units has this been seen on?

    Best regards,
    Britton

  • 0 in reply to Britton Jones
    Prodigy 20 points

    Hello Britton Jones,


    Only one unit having this issue. Can you please tell me the possible reasons for getting high impedance between AGND and PGND?

    Regards,

    Shenbagavalli S

  • 0 in reply to Shenbagavalli S
    TI__Expert 8446 points

    Hello Shenbagavalli, 

    The internal connection between AGND and PGND is handled by bond wires that go from the pins to the controller die. Over time, the bond wires can damage and even disconnect from the pin - usually as a result of the board bending under heating/cooling but the same can happen with general aging. Here it is likely a one-off case where a unit had its connection weakened after 3 years in the field. I would not be able to confirm without xray of device and potentially decapsulating the failed unit to investigate internal damage

    Best regards,

    Britton

  • 0 in reply to Britton Jones
    Prodigy 20 points

    Hello Britton Jones,
    Xray of device will be shared as soon as possible. During AGND and PGND external connection, expected output voltage is generated but 720mVp-p ripple voltage is observed. In working module, 18mVp-p is observed. Refer attached ripple voltage 720mVp-p image for your reference and solution.
    One more unit also having the same issue.

    Kindly provide root cause of this issue.

    ...

    Regards,

    Shenbagavalli S

  • 0 in reply to Shenbagavalli S
    TI__Expert 8446 points

    Hi Shenbagavalli, 

    For xray analysis, it would need to go through a formal QEM ticket at Texas Instruments for failure analysis. Without clear damage, it will be difficult for me to tell anything as that is not my expertise. Did you have a previous Field Applications Engineer supporting you when you originally built this system that can help you get a formal QEM going for these devices? 

    Best regards,

    Britton