ESD for RS-485 interface IC

[fengluan]

AS RS485 spec. Common input volatage range is (-7V,12V), How can i design my esd sturcture for signal 5V power supply.

Also for esd use, any difference between zener diode and normal nw/p+ diode?

 

[DenisMark]

Reverse biased pn-junction has low current capability, about 0.1-0.3mA/um_perimeter. So this kind of structure isn't suitable for ESD protection because it occupies sufficient die area.
Forward biased diode is the best choice for buiding current path ESD protection. It has large current capability about 10mA/um_perimeter.
To protect bus pads of RS485 IC i think u have 2 ways. The first is more easy to realize.
1) Utilize series back-to-back connected devices with snap-back action (like TFO, ggNMOS, ever ggPMOS) or chain of them if breakdown voltage of one element is too low. Regarding to polarity of ESD zap one element work as forward biased diode, other works in snap-back region.
2) Look for symmetrical middle voltage SCR device. This device occupy smallest die area, but very sensitive to process. To design such device it's requered to make and investigate a lot of experimental samples ever if u have device simulation tool with calibrated models.
Anyhow u should to contact with your foundary and receive information about ESD properties of devices (e.g. TLP measurement results). Also u need to read some books about ESD protection and look for patents.
Unfortunately i think i couldn't give u ready-to-use solution.

 

[williamy]

very good