THVD1551: Protection circuit

Section 10.2.2 says:

Figure 28 and Figure 29 suggest a protection circuit against 1 kV surge (IEC 61000-4-5) transients.

These components were chosen to protect against 1 kV surges. If you expect or test other surges, then you would choose other components. See section 10.2.1.5, and the Protecting RS-485 Interfaces Against Lethal Electrical Transients application report.

Hi Prathmesh,

So this design essentially came to be from general RS-485 best practices. The TVS diodes handle the surge current - so the diodes picked are ones that can handle up to 1kV surges (if stronger surges are expected more robust diodes may be needed) and the pulse proof resistors help prevent large pulses that occur at the time of surge but before the TVS has fully started conducting (without the pulse proof resistors the design will not be able to handle as high of voltages as damaging pulses will be directly exposed to the pins. 

The application note that Clemens has provided is a good reference for the idea of bus protection. Not all RS-485 designs need surge protection - but it is common enough that we tend to use that reference design in almost, if not all, our newer RS-485 devices. 

Please let me know if you have any other questions!

Best,

Parker Dodson