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SN74LXCH8T245: does the /OE require a pull up resistor during power/power down? If not, what are the implication / risks.

Tenyu Thach
Prodigy 10 points
Part Number: SN74LXCH8T245


Tool/software:

The datasheet for the SN74LXCH8T245 states that OE should be tied to Vcca through a pull up resistor during powerup and power down, which implies that the OE needs to be driven to GND during operation.  We experience a catastrophic failure of the device and not sure why.  Datasheet example / layout shows to simply GND the OE pin since it is active low.   Datasheet has a statement to have a pull up.   EVM shows both a pull up and pull down resistor of 10k..  Which put the OE pin halfway between GND and Vcc.. 

Could this have caused the issue? 

Datasheet also shoes an example layout where the OE is simply GNDed..  

SN74LXCH8T245 data sheet, product information and support | TI.com

Thanks,

Tenyu

  • 0
    TI__Mastermind 26440 points

    Hello Tenyu,

    We always recommend using a pull-up resistor or use a GPIO on OE pin. This way when you power-up or power-down the level translator, your I/O will be in a high impedance. Since this part also features VCC Disconnect, it's okay to have OE tied to ground. I/Os will be Hi-Z when one supply is stable while the other supply is left floating.

    In either situation, this shouldn't cause the damage that's shown in the image. 

    Looks you have 3 other LXCH8T245 next to the damaged one. Are those units configured the same? Are you able to replace the damage unit with a new one and recreate the issue?

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Prodigy 10 points

    Hi Josh,

    Thanks this is on an ATE test HIB.  Yes 3 devices are configured the same.   When the failure happened, all 4 chips stop working properly and we have replaced them on the ATE HIB.   Only one of the chips actually blew up.   The damage is located near the VccB and OE pin which made me believe that it was related to the /OE pin.  We have not been able to replicate the issue,  the level shifter is setup to translate between 3.3V and 5.0V.

    Thanks,

    Tenyu

  • 0 in reply to Tenyu Thach
    TI__Mastermind 26440 points

    Hello Tenyu,

    Are you able to monitor VCCB to make sure supply voltage does not surpass 6.5V?

    Did you experience similar issues in the past or first time observing this?

    What's connected to the output pins? Could you share the full schematic?

    Regards,

    Josh