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Ioff feature in level translators (AVCxT234 , AVCxT45) and power up/down sequence

Thibault
Intellectual 295 points
Other Parts Discussed in Thread: SN74AVC1T45, SN74AVC4T234

Hello,

 

This question may be already answered on this forum, but I haven't  seen it: On the SN74AVC1T45 datasheet, there is two information that seems not fitting together:

1)  Page 2: "This device is fully specified for partial-power-down applications using I off . The I off circuitry disables the outputs, preventing damaging current backflow through the device when it is powered down. The VCC isolation feature ensures that if either V CC input is at GND, then both ports are in the high-impedance state."

2) Page 9: "A  proper  power-up  sequence  always  should  be  followed  to  avoid  excessive  supply  current,  bus  contention, oscillations, or other anomalies. To guard against such power-up problems, take the following precautions:
        1.  Connect ground before any supply voltage is applied.
        2.  Power up VCCA .
        3.  VCCB can be ramped up along with or after VCCA"

My questions are:

- if the device can be used for partial power down and having either VCC input at GND, why is there a recommanded power up sequence?

- I found the same Ioff feature in the SN74AVC4T234 datasheet, but without the power up recommandation. Why?

 

Regards,

Thibault

 

  • 0
    TI__Expert 4705 points

    Hi, Thibault

    Thanks for raise the question here.

    Yes, you are right, both these two parts have the Ioff featur and Vcc isolation feature.

    Based on the datasheet, you can see the difference between these two parts. one is bi-directional part with DIR signal(SN74AVC1T45), the other is the single directional part(SN74AVC4T234).  Based on that , my answer is below for your questions. any unclear, please let me know.

    - if the device can be used for partial power down and having either VCC input at GND, why is there a recommanded power up sequence?

    [Junjie] Actually not following this sequcence will not damage the part or leading the part can not work well when VCCA and VCCB are on. The only concern is for the status when power up. you know the DIR logic is refenced by VCCA, put VCCA on firstly will make the DIR logic complete, so when VCCA is on, then VCCB ramp up at the same time or after, it will not lead the signal transfer direction uncertain when VCCB is ramping up.  The uncertain direction signal transfer may have the risk to lead some damage to the driver for SN74AVC1T45.

    - I found the same Ioff feature in the SN74AVC4T234 datasheet, but without the power up recommandation. Why?

    [Junjie] This part has the fix direction for signal transfer which is from B to A. there is no same concern with SN74AVC1T45.

     

    BR
    junjie