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TPS65217: TPS65217C LDO3 Failed but PGOOD signal High

Barry Barge
Prodigy 70 points
Part Number: TPS65217

We are testing a new lot of production boards that use the TPS65217C to power the AM3358BZCZ100, and on one of the boards, LDO3 has only 0.65 volts output instead of 1.8V (no apparent short on output; all other outputs are OK).  However, the PGOOD pin is high, indicating that the regulator's output is in spec.   What could cause this?

  • 0
    TI__Genius 17380 points

    Hi Barry,

    Have you looked at the output on an oscilloscope? Sometimes if a voltage regulator is oscillating it gives an unexpected DC reading.

  • 0
    TI__Mastermind 30310 points

    Barry,

    I agree with Kevin. We would need to see an oscilloscope screen shot of the voltage on LDO3 to debug this issue further.

    Since 1.8-0.65V > 0.7V, this does not sound like a leakage path through a diode, but it is difficult to say without data to analyze.

    In addition to looking at a scope shot, I would be interested to know how many PCBs you have produced and what % of PCBs are exhibiting this behavior.

    In general, once the SYS voltage is high and stable we do not hear about a lot of issues with the TPS65217 DC-DC and LDO rails. Usually, the issues are related to PWR_EN not going high and SYS dropping to around 1.7V, but this would cause other rails in your system to go low and PGOOD to be false.

  • 0 in reply to Brian Berner
    Prodigy 70 points

    There is no discernible noise on the output (less than 50mV p-p).  Output is steady 0.68V.  The first build of about 100 units did not exhibit this.  Only one of 10 tested from this second batch has the issue.  I'm sending the board back to the assembler to replace the part.  Unless I see the issue on additional units, I will assume a part failure or assembly issue.

  • 0 in reply to Barry Barge
    TI__Mastermind 30310 points
    Barry,

    Good to know. Thanks for the additional info.

    DDR3 voltages are 1.35V or 1.5V, right? Maybe there is a short to the DDR3 reference voltage (1.35/2 = 0.675 or 1.5/2 = 0.75) somewhere on the PCB?

    Looking forward to getting feedback after the results of your re-assembly of the malfunctioning PCB.