On-Chip ESD Protection for A/B Bus Ports of RS485 Using Asymmetrical dual-direction Silicon Controlled Rectifier

This paper designs an Asymmetrical Dual-Direction Silicon Controlled Rectifier (ADDSCR) device to provide on-chip ESD protection for the A/B bus ports of RS485 chips. The device exhibits a forward holding voltage ($V_{h}$) of 15.95 V and a reverse $V_{h}$ of -9.64 V, which well matches the anti-latch up requirements of the RS485 chip’s A/B bus ports (forward $V_{h} \geq 13.2 \mathrm{~V}$ and reverse $\mathrm{V}_{\mathrm{h}} \leq-7.7 \mathrm{~V}$). By combining the device’s equivalent circuit and TCAD simulation tools, the reason for the inconsistent $V_{h}$ between the forward and reverse directions of the ADDSCR has been well explained. Through adjusting the critical dimensions of the breakdown face in the ADDSCR, the breakdown voltages (BV) in both forward and reverse directions can be effectively regulated, thereby adjusting the device’s triggering voltage $\left(\mathrm{V}_{\mathrm{t} 1}\right)$. With a total finger length of $300 \mu \mathrm{~m}$, the device can achieve a forward failure current ($I_{12}$) of 28.68 A and a reverse $I_{12}$ of -28.82 A, demonstrating high robustness.