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Unidirectional TVS on I2C prevents it from working - Bi-directional one is ok

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ricko_uk:
(Updated with the part numbers and details about the voltage/length. See underlined addition)

Hi,
we have a board manufactured for years and always worked. On the I2C channel there are two separate connectors so we had two "dual TVS", one on each connector protecting both the SCL and the SDA lines. 

The original TVS were the PESD3V3L2BT for some reason bi-directional (not required because I2C only needs unidirectional). We recently changed it to PESD3V3X2UT a uni-directional and some boards stopped working but when we remove it they work perfectly.

The Ir" is running at 400KHz. The system and the I2C run at 3V3. Both connected like in the attached screenshot. The I2C and the 3V3 lines are clean and stable.
The I2C is on the PCBs only connected by a ribbon cable 3 inches long. The external I2C connector does not have anything connected to it, not even a cable.

Waiting for a PCB so I can have a look at the signals but any ideas/suggestions what could cause the intermittent faulty behaviour on the I2C channel with the new (uni-directional) TVS?
The capacitance in the new one is extremely low (less than a pF) compared to 100pF of the original, so that cannot be the reason.)

This is the PESD3V3L2BT, the original (bi-directional) which worked well for years: https://assets.nexperia.com/documents/data-sheet/PESDXL2BT_SER.pdf

This is the PESD3V3X2UT which, when used in place of the original one causes some PCBs to not be able to communicate with the I2C ICs: https://assets.nexperia.com/documents/data-sheet/PESD3V3X2UT.pdf

Thank you :)

ricko_uk:
Screenshots attached of the original and new TVS types (both connected as usual as shown)

m98:
Just shooting in the dark, could the ground potential at the diode float higher than the signal voltage for some reason? Are there some high current paths on your PCB?

uer166:
Check the footprint pinout, potentially you've flipped some pins on the TVS. If it's bidirectional, the breakdown from any pin to any other pin is >3.3V, so even if you screw up it'll keep working. No longer the case with 0.7V drop.

AndersJ:
OP is not saying which type is used when it works.
Most likely the clamping voltage is lower for the non working alternative.

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