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[FAQ] AM623: Decoupling caps on CAP_VDDSx

Michael Shklyarman
Genius 12740 points
Part Number: AM623

Champs,

Customer's asking what will happen if the decoupling caps on the CAP* nets below are not installed correctly? Will the device's behavior be well defined in this case (i.e. fail to boot etc) or would it be more sporadic based on operating mode? They are preparing manufacturing test definition and wondering about scenarios such as

  1. DNP incorrectly.  
  2. One or more of the caps are damaged due to handling and result in:
    1. open
    2. short
  3. Is there any defined behavior for wrong values  of the capacitors that can be captured in testing?

Thank you

Michael

  • 0
    TI__Guru**** 193439 points

    Hello Michael

    Thank you for the query.

    As a quick note, please refer below inputs. I have also reached out to the device experts and will add any additional comments i receive.

    Michael Shklyarman said:
    • DNP incorrectly.  
    • One or more of the caps are damaged due to handling and result in:
      1. open
      2. short

    DNP and open - i would assume to be same condition. 

    These capacitors are connected to internal LDOs that power IO bias circuits. They are required to maintain LDO stability.

    If you tried to operate the device without these capacitors, the LDO could become unstable and produce an unexpected output voltage that may damage the circuits in all IO cells associated with that bank of IOs.

    I expect the individual IO banks will not function if the CAP_VDDS capacitor is shorted to GND.

    Regards,

    Sreenivasa

  • 0 in reply to Kallikuppa Sreenivasa
    TI__Guru**** 193439 points

    Hello Michael,

    Please refer additional inputs i received from the device expert (Paul Eaves)

    The pins are connected to the output of internal LDOs that provide a bias supply to the IO cells.  I suspect the LDO will not be stable and potentially break into oscillation when no capacitor is connected, and the LDO outputs would be shorted to VSS is the capacitors are shorted. Neither is acceptable.  They must connect the proper capacitor with a low-loop inductance path.

    The explanation is in line with the initial inputs i provided.

    Regards,

    Sreenivasa

  • 0 in reply to Kallikuppa Sreenivasa
    TI__Guru**** 193439 points

    Hello Michael,

    I added additional inputs i received from the expert.

    The LDOs were not designed to accommodate a short since the only external connectivity is the capacitor. You should assume the associated LDOs were compromised even if they are working after removing the shorts.

    Regards,

    Sreenivasa