How is SM712 TVS array suitable for use with MAX485 in an RS485 driver circuit?

Question

I have an RS485 driver circuit consisting of a MAX485 driven by an STM microcontroller. I was looking at TVS diodes for basic protection from electrostatic surges from wiring and handling, and found SM712 being used everywhere.

The MAX485 is powered by a 5V supply rail, and the absolute maximum ratings of the IC specify that the Driver Output and Receiver Input voltages (A, B) should be in the range of -8V to +12.5V.

From the SM712 datasheet, the reverse breakdown voltage is 13.3V which exceeds MAX485's 12.5V. The reverse standoff voltage is 12.0V though.

I'm not use sure if I should go ahead with SM712.

I understand from reading that the standoff voltage is the max voltage at which the diode is guaranteed not to conduct. But I assume that the decision has to be made based on the breakdown voltage.

I also figured that ESD spikes will have very high voltages and the 0.8V difference between the IC's max and the diode's breakdown voltage would be crossed in a matter of microseconds. Perhaps this will not burn the MAX485?

I could use SMBJ series of diodes to get a lower breakdown voltage, but I don't have that kind of real estate on my PCB. I would like to go with SM712 and want to know if it's safe to use it.

Answer 1

Quoting from Texas Instruments' How to Choose a TVS Diode for RS-232, RS-485 and CAN Based on Voltage Ratings (sszt891):

When making a selection, you want VWM to be larger than the recommended operating region of the transceiver. As the incoming voltage goes higher, the TVS starts breaking down and conducting more current at VBR. But what’s even more important is the highest voltage parameter, VC the clamping voltage under a high-current pulse condition. VC represents the maximum clamping voltage with a specific impulse current.

When comparing VC to the transceiver parameters, you want to make sure that it doesn’t exceed the absolute maximum voltage ratings of the integrated circuit (IC). The “absolute maximum” is the maximum voltage limit that the transceiver is allowed to see at any time. Any voltage above it would put the transceiver in an unsafe region of operation, and permanent damage could result.

Vc of SM712, as specified in the datasheet:

Which makes SM712 clearly unsafe for this application.