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LMR14006: input current larger than expected

Howard Zou
Genius 12365 points
Part Number: LMR14006

Hi,

my customer has make buck circuit with LMR14006 for 5V output, input range is 9-32V.

Diodes is SS16, datasheet link is as below:

https://www.panjit.com.tw/upload/datasheet/SS12_SERIES.pdf   

They have make 2000pcs board, and find about 1% of the board input current larger than normal and the diode and inductor is over warm.

When input is 20V, the input current should be around 0.025A.

But 1% of the board the input current will increase to 0.047A, or 0.075A. Different boards have different current, but all are larger than normal.

Could you please suggest how should we check the problem? What waveforms are needed?

  • 0
    TI__Mastermind 25590 points

    Howard,

    Please share schematic, layout, and waveforms of SW, CB, VOUT, and VIN.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Genius 12365 points

    Hi, 

    the schematic and PCB is shown below:

    with good board, 0.025A input current, the waveforms are:

    yellow-Vin, green-CB, purple-SW, blue-5V out

    with bad board 0.075A input current, the waveforms are:

    yellow-Vin, green-CB, purple-SW, blue-5V out

  • 0 in reply to Howard Zou
    TI__Mastermind 25590 points

    Howard,

    The Schematic looks good. The layout looks good with one comment. The small input cap could be rotated 180 degrees with a GND trace routed out the right side of the device to reduce this loop area. It may be worth a test to modify a bad board with this new cap arrangement to see if it helps.

    The waveforms show me that the device is switching from DCM frequency foldback mode to CCM to compensate for the increased load, presumably being dissipated in the inductor and diode. But I'm not sure how those waveforms would cause the diode or inductor to overheat.

    How much hotter are the inductor and diode on a bad board compared to a good board? And does this change with load?

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Genius 12365 points

    We don't have the accurate number of temperature, with good board, the device could not be felt warm by hand(<35C). With bad board, the device is too hot to be touched(>50C).

    We also remove the bad IC and compare each pin to GND resistance with the good IC.

    And we can see significant difference of the resistance between pin 6(SW) to GND.

    With good device, the resistance is around 3MOhm.

    With bad device, the resistance is just 0.189kOhm.

    So I guess maybe it's soft failure? Although the waveform is good, but the device has failed somewhere.

    Any suggestion how to analyze this kind of problem?

  • 0 in reply to Howard Zou
    TI__Mastermind 25590 points

    Howard,

    I agree, sounds like the part is getting damaged somehow.

    Are any of the pins violating abs max at any point during operation?

    Also you mentioned at first that the diode and inductor are warmer. But you've also mentioned the IC is warmer. Are all 3 warmer?

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Genius 12365 points

    Samuel,

    The diode is the hottest part.

    So we are wondering if the diode's abnormal(for example quality issue) will affect SW pin of LMR14006?

    Do you have any insight what kind of abnormal of the diode would lead to SW pin break?

    For the waveform for higher input current 0.075A, it seems that SW be below -1V. Is it purely caused by the diode?

    But we are not sure "SW below -1V" is the cause or the result since good board SW won't be below -1V.

  • 0 in reply to Howard Zou
    TI__Mastermind 25590 points

    Howard,

    Yes a damaged diode could be the reason the diode gets hot and the SW pin gets damaged. Try depopulating the diode and check it against a good diode. Is the forward voltage the same? Is the reverse breakdown the same? Is the leakage the same?

    A damaged/broken diode would be the cause of the SW voltage going lower than typical. This can damage the IC and cause the diode to overheat.

    The diode could get damaged from violating abs max in the diode datasheet. It's rated for 60V and there's only 32V in the input. Though the input cap loop is large so the SW ringing may be excessive. Check the SW node with a 500MHz+ scope without bandwidth limit to see if there's excessive ringing.

    -Sam