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TPS709-Q1: fault analysis

Part Number: TPS709-Q1

HI  ti

     On board etc equipment is using tps70936qdbvrq1 for 5V to 3.6V,;

      It is found that one prototypes have abnormal current, and the normal working current is 20~50UA, but the fault prototype reaches 87mA. Finally, it is found that the chip of tps70936qdbvrq1 is abnormal.

      Please help to see if there are problems in the schematic diagram and PCB design. Thank you。

Test comparison results of LDO good and bad parts:

Abnormal LDO: PIN3 impedance to ground 70 Ω, The output voltage is high, above 3.8V.

Normal LDO: the measured impedance of PIN3 to ground is about 10m Ω; The output voltage is normal, about 3.6V.

  • Hi Rain,

    I don't see anything wrong with the schematic or layout. 

    This failure mode sounds like the EN pin has been damaged. Is there a possibility of the VCC_5V supply overshooting during startup and causing an overvoltage on EN?

    For the bad LDO, you say the output is above 3.8V. How far above 3.8V is it? Is it close to VIN = 5V?

    Is this an isolated incident or is there more than one prototype board that this behavior has been observed on?

    Have you try replacing the bad LDO with a fresh one?

    Thanks,

    Nick

  • HI Nick

            1.  VCC_5V supply overshooting during startup,I will verify whether this situation exists;

             2. For the bad LDO, the output is above 3.82V ;

             3.  Have two prototype board that this behavior has been observed on;

           4. After replacing the bad LDO with a fresh one, It is OK.

  • Hi Rain,

                 Nick is out on timebank. He will get back to you in 2-3 business days. Thanks!

    Regards,

    Srikanth

  • OK  thanks

    add :  Is there a possibility of the VCC_5V supply overshooting during startup and causing an overvoltage on EN?-------------->Both normal and abnormal equipment have been tested for hundreds of times, and the maximum voltage peak is 5.5V

  • Hi Rain,

    Since replacing the abnormal LDO fixes the problem, this makes me think it incurred some damage at some point, and if it was not damaged during startup then that makes me think it must have been damaged during manufacturing. Do you have a way to use a separate EN voltage signal on one of the bad units to see if the EN circuit is working? 

    Thanks,

    Nick