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TXU0104-Q1: Level shift IC showing VCCA and VCCB power leakage

Philip Pun
Prodigy 30 points
Part Number: TXU0104-Q1

While probing the Ti level shift ckts between 1.8v and 3.3v rails, the 3.3v to 1.8v signals showed the output voltage is higher than 1.8v. Its likely that my probe placement slipped and short circuited something in one of the IC. Instead of 1.8v at the VCCA supply, the board is now measuring 3.3v. There're four 3.3v to 1.8v level shift IC and many 1.8v to 3.3v level shift in the board. But I can't tell which chip is creating the short because there's not sufficient heat to highlight a hot spot for IR detection.

The VCCA to GND measured at 2k ohm instead of a much higher impedance from a good board. VCCA to VCCB still show a high resistance so its not a hard short.

  • How can I tell which TXU chip has gone bad?
  • What damage can create this current path between the VCCA and VCCB rails for this IC?
  • 0
    TI__Mastermind 42175 points

    Hi Philip,

    • Unfortunately, by doing ABA swap and isolating suspected units as there is not an easier way to tell exactly where the failure originates from. Swapping out the suspected units with known good units to verify i.e units with output voltages that do not match VCCB voltages.
    • Damages are typically caused by exceeding the absolute max ratings of the device, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 30 points

    Since we don't know which one of the IC is causing the leakage path, we'll have to remove the device one by one until the problem is removed. But this is somewhat difficult to rework the PCB.

    Is there a failure mechanism how this can damage the circuit?

  • 0 in reply to Philip Pun
    TI__Mastermind 42175 points

    Hi Philip,

    Yes, please help use the below draft as a failure mode distribution reference, thanks.

    SFFS787_reviewed.pdf

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 30 points

    The functional block diagram show the OE input is part of VCC(MIN) power domain. Datasheet table 6-1 show OE pin can be pull to VCCA or VCCB and section 9.3.2 show the pin has over-voltage tolerant. In our PCB, VCCA = 1.8v and VCCB=3.3v or vice versa. The OE pin is pulled high to 3.3v and not VCC(min) after the board powers up. Is this ok and not damage the chip for normal operation?

    Is there a recommended soldering rework or SMT soldering profile we should follow to remove and replace the chip in our PCB?

  • 0 in reply to Philip Pun
    TI__Mastermind 42175 points

    Hi Philip,

    Yes OE can be tied to either VCC but the OE's electrical and switching characteristics would be for Vcc_min, 1.8 V in your case - for normal operation.

    With OE pulled to 3.3 V, it just means the enable and disable time would be based on the below.

    If OE is shorted to GND, all I/Os will be in Hi-Z, this do not seem to be your case as you see outputs toggling.

    Philip Pun said:
    Is there a recommended soldering rework or SMT soldering profile we should follow to remove and replace the chip in our PCB?

    No specific recommendation. Just remove the device that is observing 3.3 V at the outputs when VCCA=3.3 V and VCCB=1.8 V and replace with a known good unit. If known good unit sees 1.8 V, suspected device is most likely damaged. If 3.3 V is observed, then your concern is system or configuration related and would further recommend schematics and waveforms for additional debugging, thanks.

    Best Regards,

    Michael.