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SN74AVC2T45: SN74AVC2T45 False Output at Power Up

Connor Schutte
Prodigy 60 points

Part Number: SN74AVC2T45

I am using the SN74AVC2T45 to provide electrical isolation between GPIOs that connect from a CPU to an STM32. The below photo shows my setup

image.png

In this setup, 3V3_STM is enabled first, and then after a button press 3V3_RUN gets enabled. When 3V3_RUN is enabled and ramping up, the A1 and A2 outputs of the level shifter seem to have a slight ringing that causes a false triggering at the STM32. The below o-scope capture shows the A2 output in yellow and the B2 input in blue:

SCR11.PNG

Measurements showed that this happens every enabling of 3V3_RUN when 3V3_RUN has reached 600mV. I added a 1nF capacitor in parallel to R342 to see if this would tamp down the ringing, and it led to this output where the output A2 is in Green, the input B2 is in orange, and the 3V3_RUN power is in yellow:
Screenshot_2025-10-17_2_011137.png

It seems to me that this output is caused by the level shifter attempting to drive the output high by default after it sees VCCB reach an enable threshold, despite the input being pulled low through the entire boot up process. I have tried replacing the 100k pull-down at the DIR pin to a 0 ohm to see if this would fix ti, but this created no change.

Is this the correct operation of this device? Is there any way to prevent this from happening during boot up?

  • 0
    TI__Mastermind 41190 points

    Hello Connor,

    Since this part is enabled/active at all times, we need to make sure the inputs buffer (in this case VCCB) is powered up first before the output buffer (VCCA).

    Your description to the possible cause is correct. Once VCCB is stable and VCCA begins to ramp up, there will be a "glitch" on the output pins when VCCA reaches the enable threshold.  

    There are two ways to fix the unwanted glitch:

    1) Ramp VCCB before VCCA

    2) Switch to the SN74AXC2T45. The AXC2T45 is pin-to-pin with AVC2T45 and has an improved POR circuitry which eliminates unwanted output glitches.

    Regards,

    Josh 

  • 0 in reply to Joshua Salinas
    Prodigy 60 points

    Hi Joshua,

    Ramping VCCB before VCCA goes against the recommendation in the datasheet so instead I replaced the part with the SN74AXC2T45 and found that this fixed the problem. I will proceed with the AXC2T45 part for my design.

    Thanks,
    Connor