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On my board comes a UART line with a voltage of 3.3V. To safely communicate with the controller on 5V, I use a level offset based on BSS138 transistors. My goal is reliable ESD protection of the UART line from the 3.3V side.

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For this, I want to connect RX_L and TX_L to ground using TVS diodes of low capacitance. The problem arose in the choice between unidirectional and bidirectional diodes. Doubts arose because I learned that the ESD voltage can be of different polarity, in some case a bipolar TVS could miss a pulse of reverse polarity and cause damage to the device. Tell me, is that so? What type is used for UART lines?

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Unidirectional voltage suppressor diode with clamping voltage greater than 3.3 V is sufficient. For the positive spikes, the ESD diode will kick in into action soon after the voltage reaches the clamping voltage. For negative spike, the TVS diode acts as normal diode shorting the signal soon after the forward voltage of the diode is crossed. SMF series from little fuse has reverse stand off voltage starting from 5 V. General purpose ESD protection diode example is below: SP3222 0.9pF 30kV dual channel TVS for example can be used.
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  1. Adding a small capacitor between the line and the ground also helps.
  2. Also consider adding a series resistor of a few 100 ohms depending on the baud rate of the UART. This will also helps in limiting the current value during an ESD strike.
  3. The resistor and capacitor value should be such that the 5*RC should be atleast 10 times smaller than the bit rate of the UART communication.
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  • \$\begingroup\$ Thanks for the help! I found one TVS diode, its capacitance is 600pF. UART 115200 baud rate, is the capacity of this diode too high? \$\endgroup\$ – Алекс Гарисон Aug 22 at 7:31
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    \$\begingroup\$ 115200 has about 9 usec of bit interval. if we consider 20% of the bit interval it will be 1.5 us. hence, the 5*RC value cannot be higher than 1.5us. Even if i consider nomianl line resistance of around 1 ohms, the timing will be then 5*1*600p that is 3 us. mraginally okay but not good. pelase choose a value lesser than 50pF or less. \$\endgroup\$ – Umar Aug 22 at 12:26
  • \$\begingroup\$ lcsc.com/product-detail/… I found a great option on several channels. Capacitance <0.6 pF, low clamp voltage and high current. I hope this is useful to someone) \$\endgroup\$ – Алекс Гарисон Aug 25 at 7:49
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    \$\begingroup\$ @АлексГарисон it also means there can be one series resistor on the line limiting the current in case of an ESD event and also helps in keeping the signal edges smoother \$\endgroup\$ – Umar Aug 25 at 14:04

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