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TPS65131: Noise issue with TPS65131

Part Number: TPS65131

Hi...

One of my customer is using TPS65131 for 5V input , +/-15V output configuration. They have an application where load current is very small (most of the time load will be 2mA & during relay drive +15V draws 200mA for 100mS).

With the fresh board or fresh device on the same board, design is working fine & no issue observed. Over a period (running for 8 to 9 days), part behaves strangely & output develops the noisy output (ripple of 1.25MHz - looks like switching ripple). Customer has tried with both forced PWM & pulse skipping but not improving.

Pls. find the attached waveforms & schematic.

Pls. help resolving the same.

Mitesh

  

  • Hello Mitesh,

    If the design is working initially and degrading over several days of operation, I would suspect some passive component issues like capacitors degrading with applied DC voltage. Or some board assembly issues. Schematically, the design looks good but I do have following comments:

    1. The buck-boost diode used in the design (D3) is rated for 20V reverse breakdown voltage and in the application with 5V input and -15V output, the voltage across this diode can be slightly over 20V. So I suggest to use a diode with slightly higher reverse breakdown voltage like MBRM130LT1G to provide some margin.

    2. For better ripple performance, I recommend to ensure that PSN/PSP pins are pulled down to ground i.e. R18/R19 are installed on the board and R16/R17 are not installed.

    Kind Regards,

    Liaqat

  • Liaqat,

    We have asked customer to change the diodes as you mentioned but I am curious to know that why any new part on same board is working fine. Once they are observing the noisy output they simply replacing the device without changing any other component & output shows normal behavior. 

    After certain days of operation this issue start developing gradually. What could be the reason? Are diodes creating part's gradual degradation?

    Mitesh

  • Mitesh,

    My apology as I missed the point that on a malfunctioning board, when customer installs a new TPS65131 device without changing anything else on the board, the same board starts to work well for few days. This is very strange behavior and, assuming everything described above in your post is correct, the only conclusion I can draw from this is that the batch of TPS65131 devices that customer has is defective. I have not encountered this kind of behavior with customer applications in the past. Schematically, the design is correct and, although I doubt that layout can be the issue here, I would be happy to review the layout to see if I can spot something.

    The other suggestion I have is to get an EVM board and swap TPS65131 devices between EVM board and customer's malfunctioning board to see if the device from EVM board once mounted on the customer board still behaves in this problematic fashion or not. Also, if the device from malfunctioning customer board continues to behave in a problematic way on the EVM board or not.

    My comment about the diode was not necessarily to address this particular issue but rather something that I noticed on the schematics that should be corrected regardless of this ripple/noise problem.

    Kind Regards,

    Liaqat