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I want to protect sensors that use 3.3V I2C from strong ESD. After searching internet for a while I found that LCO3-3.3 provides protection from 30kV. However, I'm still not sure if the one can be used for protecting 3.3V I/O.

(1) Its max. stand-off voltage is 3.3V, and min. breakdown voltage is 3.3V. Would it mean that 3.31V or 3.32V will be clamped and the I2C communication will be interrupted?

(2) max. clamping voltage is 17V. Then would my 3.3V I2C device be broken by ESD due to the still high voltage after being clamping?

(3) Between uni-direction and bi-directional TVS diode array, which is appropriate for I2C port protection?

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  • \$\begingroup\$ you want to have series R between pins 1,8 and 4,5. Some are like that, eg some USB protectors. \$\endgroup\$
    – Henry Crun
    Jun 28, 2018 at 12:23

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1) The max standoff voltage is a guarantee of no conduction below that threshold. So there will be no clamping below 3.3V. Above that and the diodes will begin to clamp. Do not think of the TVS Diode as a switch that is on or off, it is a semi-conductor so it can partially conduct.

2) Max clamping voltage is the voltage at which the TVS Diode can conduct the most current. On the datasheet, clamping voltages are given for 50A and 100A and the graph, Figure 4 shows this over its working range. This is shows for an 8/20us pulse which is very fast, typically ESD events from human contact are slower (see this RS article on discharge rates. They spec to a 153ms event.). What this means is that the rate of change of current (di/dt) is lower, so the clamping voltage is lower.

3) I2C is a duplex communication protocol. Duplex means that it has two data lines, but they only operate in one direction each. Duplex Comms The question about unidirectional and bidirectional can lead to a confusing answer, as admitted in this ON Semi paper. Best to deal with single diodes as a beginner.

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    \$\begingroup\$ Thank you for the clear answer! So according to the On Semi paper linked here, bi-directional TVS is needed only for AC power or the signals that include both positive and negative voltages. So for I2C and SPI, uni-directional diode is good enough. Is it correct? \$\endgroup\$
    – Nownuri
    Jun 28, 2018 at 13:19
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    \$\begingroup\$ Yes, you can simply use unidirectional diodes across the data lines and ground. \$\endgroup\$
    – MIL-SPEC
    Jun 28, 2018 at 13:29
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    \$\begingroup\$ My system needs to have a floating ground. So ESD can occur when the system ground touches large metal. In such case, could a negative voltage applied to the signal lines..? \$\endgroup\$
    – Nownuri
    Jun 28, 2018 at 13:36
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    \$\begingroup\$ Of course. Bi-directional diodes would protect from this. \$\endgroup\$
    – MIL-SPEC
    Jun 28, 2018 at 14:15
  • \$\begingroup\$ Statement 3 is incorrect. I2C is half-duplex. I2C has a clock and data signal that are both inherently bi-directional signals. They are not as you depict as TX and RX. Also, bi-directional TVS is really only necessary in cases where the operating region of the signal is both positive and negative. since I2C is likely operating from 3.3 to GND then a unidirectional should suffice. \$\endgroup\$
    – busfault
    Jun 2, 2021 at 21:09

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