Analysis And Design Of Rs485 Interface Chip TVS Devices

At present, smart grid becomes one of the important measures of energy conservation and emission reduction in the world, which has long distance communication in RS485 network and is a key transmission interface in smart grid. The RS485 transmission bus general frame in the outdoor or along the cable laying, it can easily be damaged by transient interference which caused by lightning. In addition, RS485 network generally use the Party-Line(or Bus) structure, which a bus mounts dozens to hundreds of RS485 transceiver, so the high voltage surge caused by lightning and surge can lead to hundreds of RS485 transceiver damaged, and then lead the network to paralysis. Therefore, the protection of surge voltage is an integrant matter for RS485 chip designing, but also the important measures to improve the system reliability and safety.As to the traditional RS485 transceiver, in order to comply with the DL/T645 standard, the transmission line port A, B will have a HBM specification of 15 kV, but in practical application, such protection is not enough. At present the general solution to prevent the surge voltage is plus transient voltage suppression diode(TVS) outside the RS485 transceiver. But the independent TVS device is high-cost and inconvenient. As well, the varistor has a large parasitic capacitance and conduction resistance, the low-voltage varistor has a large leakage current, so the varistor is not suitable for RS485 interface protection. The TVS device which making by the conventional semiconductor process has a fast reaction speed, but it unable to withstand the high voltage and high current, so it is difficult to resist the impact of lightning surge. However, using the new CMOS IC technology can overcome many bottlenecks on technology. The main work of this paper is as follows:First, integrate a TVS device into the RS485 transceiver, to solve the disadvantages of varistors for its large parasitic capacitance and conduction resistance, and to overcome a lack of ability of traditional semiconductor fabrication TVS in withstand the high voltage and high current deficiencies.Second, the main ideal of this paper is: based on the application guidelines for TIA/EIA-485-A, and design an RS485 transceiver by using Macronix 0.5μm CMOS technology and Cadence Virtuoso EDA tool. The transceiver was implemented the monolithic integration of TVS device. The chip adopts bidirectional communication, half duplex, and has the characteristics of lightning-resistance, surge-resistance, low power consumption, short circuit protection and so on. The input impedance of chip is 1/8 unit load, so the transceiver has the capable of driving 256 transceivers at the same time.The design of the chip is mainly composed of a receiver, driver and TVS device. The RS485 transceiver is in manner of balanced driving and differential receiving. The RS485 transceiver has implement the pre designed index through thrice tape-out optimization and improvement, and passed the engineering test of performance and reliability. Compared with the domestic similar products, the transceiver designed in this paper has a smaller area, and has the characteristics of stronger anti-interference ability.