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TMS320F28375S: Controller failure

Naveen Kumar reddy Nelavoy
Intellectual 500 points
Part Number: TMS320F28375S


Dear team,

We are using TMS320F28375S controller in our board and we have observed three failures of controller where VrefHIB pin shows  low impedance.

1)Following are the impedances measured from each pin to thermal pad i.e Digital ground in one of the controller failed

VddA - pin38 - 21.2 Ohm

VddA - pin18 - 24.7 Ohm

VrefHIA - pin19 - 115.3 kOhm

VrefHIB - pin37 - 5.5 Ohm

We assume that VrefHIB is source of failure.

2)In one controller all ADC channels related to ADCB are deviated from their actual values.

We are unable to find out the reason for failure to initiate a corrective action. Could anyone help in resolving this issue.

  • 0
    TI__Guru 62975 points

    Naveen,

    There isn't any obvious issue that comes to mind based on the limited information.  If you have isolated this to just the VREFHIB pin, I would recommend checking the software settings for the ADC-B reference and confirm that the external reference voltages are not applied to the VREFHI pins before the device (specifically VDDA) is powered on.

    -Tommy

  • 0 in reply to tlee
    Intellectual 500 points

    Hi,

    Please find following clarifications

    1. I wasn't sure on which software parameters to check. Can you specify any register value which you need

    2. We have observed that VREFHI rises to 1V in 120usec before VDDA is powered up. So this 1V difference is maintained for almost 300usec after which both are equal. Even we have suspected this and kept power cycle test for controller power supply. We did almost 1,00,000

    3. In the datasheet it was mentioned not to load  VREFHI externally, but we have loaded with 1.8K resistor. Will this be a problem

    I could observe only above deviations and request you to clarify on this

  • 0 in reply to Naveen Kumar reddy Nelavoy
    TI__Guru 62975 points

    Naveen Kumar reddy Nelavoy said:
    1. I wasn't sure on which software parameters to check. Can you specify any register value which you need

    Please disregard.  I was thinking of another device.

    Naveen Kumar reddy Nelavoy said:
    2. We have observed that VREFHI rises to 1V in 120usec before VDDA is powered up. So this 1V difference is maintained for almost 300usec after which both are equal. Even we have suspected this and kept power cycle test for controller power supply. We did almost 1,00,000

    The effects would likely vary from device-to-device and board-to-board.  The primary concern is that power is shunted from VREFHI to VDDA through ESD protection diodes when VREFHI > VDDA.  This also applies to the ADC input pins as well.  If there is sufficient exposure to this condition, the diodes and nearby structures may suffer physical damage.

    Naveen Kumar reddy Nelavoy said:
    3. In the datasheet it was mentioned not to load  VREFHI externally, but we have loaded with 1.8K resistor. Will this be a problem

    The SAR ADC uses VREFHI directly for conversions so disturbances on the VREFHI level may influence conversion accuracy.  The no-load guidance is generalized to minimize the potential for VREFHI disturbance.  The 1.8kΩ resistor is acceptable as long as the VREFHI pin level is stable and the ADC conversions meet the performance requirements of the system.  VREFHI capacitor sizing and placement can help to mitigate disturbance in this scenario.

  • 0 in reply to tlee
    Intellectual 500 points

    Hi,

    On every failed controller , we have observed apprx 18Ohm impedance from VDDA to GND pins, so from this data can we relate to the failure reason is because of VREFHI > VDDA. I mean can we prove this by performing a test on the board which results in failure of the controller

  • 0 in reply to Naveen Kumar reddy Nelavoy
    TI__Guru 62975 points

    Naveen,

    I suppose you could try to provide a static VREFHI > VDDA voltage on a good device to see if it begins to fail like a bad device after some amount of time.

    What is your observed failure rate?  Do you have an assigned TI sales representative?

    -Tommy

  • 0 in reply to tlee
    Intellectual 500 points

    Hi,

    1. As VREFHI > VDDA for 120usec, to simulate the condition we have used power cycles as test procedure in which there is no failure observed. But if we do a static test it might fail but can we relate to our failure scenario

    2. Failure rate is 3 out of 30 controllers

    3.I will check with our purchase team regarding our sales representative and may i know the purpose. Is it to submit the failed controller, so that you can review

  • 0 in reply to Naveen Kumar reddy Nelavoy
    TI__Guru 62975 points

    Naveen Kumar reddy Nelavoy said:
    1. As VREFHI > VDDA for 120usec, to simulate the condition we have used power cycles as test procedure in which there is no failure observed. But if we do a static test it might fail but can we relate to our failure scenario

    2. Failure rate is 3 out of 30 controllers

    I suppose it would make sense to compare the power sequences of a bad system versus a good system.  Maybe the external VREF ramps faster or VDDA ramps slower on a bad system.

    Naveen Kumar reddy Nelavoy said:
    3.I will check with our purchase team regarding our sales representative and may i know the purpose. Is it to submit the failed controller, so that you can review

    Yes, I was thinking that factory evaluation of the bad devices might provide some additional clues.  However, I can imagine that the return request might be rejected without some additional push from the sales team.