Damage on ST485B component

[Heineraf]

Hello,

Good day!

We have a RS485 communication system and we are usign the ST485B, but we recently had an issue with a couple of PCBs(attached picture)

We are using an external DC/DC converter as 5V power supply (attached schematic). I followed application guide to create the schematic

The component was damage in a very bad way, near of the power supply node, please could you review and give me some recommendations.

Best regards,

Heiner

[Peter BENSCH]

Welcome @Heineraf, to the community!

Although older transceivers such as the ST485B do not yet have ESD protection built in, the photo shows anything but the kind of simple ESD damage that can be caused by touching a charged human body. The destruction at U4 and the wide smoke trail rather indicate a rather large amount of energy that either came in via A and B or via VCC.

Although you have AC terminated A and B via C1, C25 and C26, you have not added any TVS, which I strongly recommend. You can use the following configuration, for example:

• between A (pin 1) and B (pin 2): ESDALC8-1BF4
• from A and B to GND: ESDAVLC6-2BLY

Tips:

• there should be a sufficiently large GND plane under the connectors and the TVS, to which the dual TVS with several vias is connected (lower inductance)
• the ESD protection should be as close to the connectors as possible in order to eliminate the transients right there

It may also be useful to connect a 5V TVS between VCC and GND.

Hope that helps?

Regards
/Peter