Output Port Protection of the ADM3251E

Hi Lijie,

Topologically, this protection circuit will protect against surges up to the limits of the gas discharge tube; however, the particular TVS diode selection is inappropriate for the ADM3251E. 

The SMBJ26CA's trigger voltage is approximately 32V @1mA and its maximum clamping voltage is 42V@14A. The TBU will limit the transient current to its trigger current of approximately 200mA, so the SMBJ26CA's working clamp voltage will be someplace between 32 and 42V.

The ADM3251E's receiver's R_IN ABSMAX is +/-30V and transmitter's T_OUT ABSMAX is +/-15V and so the transient voltage the SMBJ26CA will allow will exceed the ABSMAX for the ADM3251E likely resulting in damage.

To properly protect the ADM3251E's receiver, I'd suggest using the SMBJ18CA which has a reverse standoff voltage of 18V  (greater than the RS-232 spec of 15V) and a max clamping voltage of 29.2V which is less than the 30V ABSMAX.

For the transmitter, the normal output swing is +/-5.7V and the typical output short-circuit current is 12mA (which is less than the RS-232 specification that it is to be less than or equal to 100mA).   Since the TBU's trigger current is ~200mA, it will not trigger when there is a line short and will only trigger when there is an external surge.   Since the transmitter is supposed to be the only generator on this circuit, we can choose a TVS based on its ratings and there are many different options.   I'd suggest the SMBJ8.5CA which has a reverse standoff of 8.5V and a max clamping voltage of 14.4V. 

Eric