On the STM32 Nucleo-144 eval board, there are 3 different ESD protection devices for the USB port:
U19 = USBLC6-2SC6
U21 = ESDA7P60-1U1M
U22 = ESDALC6V1-1U2
Source: https://www.st.com/en/evaluation-tools/nucleo-h745zi-q.html#cad-resources
Can anyone give information on why these are chosen, especially, why there are two different types chosen for the VBUS (voltage supply) and the ID signal protection? How necessary is it to have this exact types?
U19 is for high speed differential data. It does not need to handle a lot of energy as data lines are the last ones to make connection, but it must be suitable for fast signals.
U21 is for high power transient voltage suppression. It does not need to handle fast signals but it needs to handle a lot of energy as power wires make connection first.
U22 is just for extra protection for the MCU GPIO. It does not need that much power handling and there is no high speed signals. The package is extremely small so it does not take a lot of space.
And no, it is not necessary to have exactly these types, but the protection devices are selected based on their role and how much protection you want and from what.
