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LMX2531LQ1312 debugging faulty board

Warwick Barnes
Prodigy 20 points


I have one board in a batch of 12 that has a problem with the LMX2531LQ1312, the only sign of a problem other than I do not get any frequency output from the Ic is that the "Vreg Buf" signal on pin 6 is low when it should have voltage on it.

I have checked this pin for short circuit to ground, had the board X-rayed and the chip replaced, still no joy.

The data sheet does not have enough information on it to detail why this signal might be low when all the other PSU pins with there decoupling caps are OK.

Is there a more detailed block diagram of the internals of this chip so I can work out what is causing this one board to fail?

  • 0
    TI__Mastermind 28021 points

    Warwick,

    It is hard to tell what is causing these issues, but it sounds like it might be powered down.  Here are some diagnostics:

    1.   If part is powered up, it should be drawing around 30 mA and the OSCin pin should have a bias level around 1.5 volts or so.  The VregVCO pin should have a DC bias level around 2.65 volts.  The VregPLL regulator is closer to 2.5 volts.  If you measure these and they are all around 0 volts, it sounds to me like the device is powered down.  Not only can the CE pin do this, but if it is improperly programmed, this could be causing the issue.

    2.  The package is asymmetrical, so sometimes this can bring up soldering issues.

    3.  There are MSL (Moisture Sensitivity Level) guidelines for this part.  If these are not followed, then this might cause some issues.

    4.  The DAP on the bottom of the device should be grounded.

  • 0 in reply to Dean Banerjee
    Prodigy 20 points

    Dean,

    Thanks for your response

    I will check these votages, I don't think it is something left unprogramed as I have 20 boards that work.

    But I will check the voltagepaying more attention to the actual level.