TMUX7236: S/D line protection

What protection components do you already have? Please show a schematic.

When users can touch the lines, you definitely need ESD protection. (And RS-485 system commonly use strong TVS protection because they are used in industrial environments. Is this the kind of environment that your application will be used in?)

What is the range of signal voltages that can actually happen on the bus (i.e., can there be ground shifts between the nodes)? The simplest solution might be clamping diodes to the power rails.

Thank you for the fast reply.

Ground shifts are always possible, but should be limited to potential difference on cable due to noise. For range of signal voltages: the user has direct access to it, so would be nice to have some over and undervoltage protection.

For a big clamping diode, there is not a lot of space. This one is currently in use and during stress testing on bench has been fried. A buffer circuit would be prefered but for the baud rate of RS485, none where found.

To reduce the current through the clamping diode, put a small series resistor between the diode and the bad voltage. (10 Ω pulse-proof resistors are commonly used in RS-485 systems; if you already have on near the transceiver, move it here.)

I'm afraid of what influence this would have on the Analog input (other usecase of this mux)

Hi Tim,

For RS-485 we typically recommend ESDS552. It's a 2-ch diode with a 12V Vrwm so it can support the typical RS-485 common mode range.

This will protect from any positive/negative transients seen on the data lines. 

Regards,

Sebastian 

How much current flows through AI1?

Hi Sebastian,

Thank you for the diode suggestion! I will cetainly try to implement it!

We are currently conducting some tests on a very simular test setup, I will let you know the results.

Hi Tim,

When you mention overvoltage, do you mean a DC overvoltage? or transient events like IEC 61000-4-2 (ESD) and IEC 61000-4-5 (Surge)? 

Regards,

Sebastian 

Esd is already protected by the first mux no? For general, we also want to protect DC overvoltage.

Thank you

Hi Tim,

It depends on what IEC ESD robustness level you're looking for. Any overvoltage protection would need to be internal to the mux or with another IC, ESDS552 won't be able to clamp DC overvoltages over it's breakdown voltage. 

Regards,

Sebastian