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Amazing Provide the most Robust RS485 Transceiver with Surge TVS Embedded2013/01/28


Amazing Provide the most Robust RS485 Transceiver with Surge TVS Embedded
AMC R&D / Interface IC Team
The RS485 has special features in terms of noise immunity, wide common-mode voltage range and multipoint laying capability. Therefore RS485 is a key communication interface in the smart grid network for the worldwide green policy.
The RS485 network is commonly configured as bus-Line or daisy-chain which connects several dozens or hundreds of the RS485 transceivers by the long twisted cable. The bus line is often explored outdoor and even putted alone with the power cable so that the RS485 devices are easily suffered from the ESD and surge events. Once the surge/ESD event happens, most of the RS485 transceivers connected to the network cable will be damaged. It is very important to take surge protection into consideration to enhance the stability and reliability of RS485 network communication. Amazing has successfully developed the surge-self-protected RS485 transceiver IC, AZRS3082, which has already integrated the robust surge TVS on the bus pins. AZRS3082 is the most robust surge-protected RS485 transceiver on the current market, which against not only system-level ESD (IEC 61000-4-2, Contact Discharge) to ±30 kV, but also the lightning surge (IEC 61000-4-5, 8/20µs) to ±30 A.
Amazing RS485 Provide the Most Powerful ESD Protection and Surge Protection
The ESD capability of the traditional RS485 transceiver should meet DL/T 645 standard, the bus pins (both A and B) have the specification of the HBM 15kV ESD protection. But the HBM ESD protection only guarantees a Component-Level ESD, which is not enough to meet the System-Level ESD and lightning surge requirement for real application. To solve the System-Level ESD event, the on-board ESD protector such as TVS (Transient Voltage Suppressor) is absolutely necessary and must be placed on the PCB exactly to efficiently protect the RS485 transceiver. For the surge protection issue, the general ESD TVS with high turn-on speed is not stronger enough to against the surge energy. Although the Surge TVS is robust enough to protect itself but can not efficient protect RS485 transceiver on the fast ESD transient. Therefore, the whole-system ESD/Surge off-chip protection is two-stage design, the first stage is the ESD TVS near the RS485 transceiver and the second stage is the Surge TVS near the I/O port.
On the current application, there are three type of surge protector: gas discharge tube, varistor and TVS. The gas discharge tube has very strong surge current capability, but the characteristic of the both slow turn-on speed and high clamping voltage can not efficiently protect the RS485 transceiver. So that gas discharge tube can not design single alone for ESD/Surge protection. The varistor is low reliable component for Surge protection design due to the junction leakage problem. By the way, the varistor has the characteristic of both the larger parasitic capacitance and the high clamping voltage so that the device. Therefore, the drawbacks of varistor are not acceptable for system design by now.
Single TVS device is fabricated by CMOS process, is called “off-chip TVS”, has high response time for ESD event but it still poor for high energy surge event by now. So as to the additional off-chip Surge TVS is indispensable to protect core IC. Two stages ESD/Surge design should putted one ESD TVS and Surge TVS on the PCB, which consumes much PCB space and cost. Moreover, the PCB should be redesigned and rework for several times to put this two TVS devices. The best solution is to integrate the ESD/Surge TVS on the RS485 transceiver inside.
The Surge/ESD TVS devices integrated in the functional IC is called “On-chip TVS”. In CMOS Technology, the On-chip Surge TVS is difficult to integrate with the functional IC due to the process control. Amazing has successfully developed the On-chip ESD/Surge TVS and combined with the RS485 transceiver IC on the same process.
The On-chip ESD/Surge TVS has been featured by the Transmission-Line-Pulse (TLP) curve of the AZRS3082, as shown in Fig.1. The low clamping voltage 23.0V under 17Amp is the important key parameter for on-chip ESD/Surge TVS device. The Surge characteristics of the peak pulse current is compared AZRS3082 with competitor as shown in Fig.2. By the Fig2, the lower clamping voltage of the AZRS3082 can sustain higher peak surge current than competitor. Moreover, the maximum peak current for AZRS3082 is 42.0Amp, which can sustain system surge level 2.
The detail system level ESD specification for AZRS3082 is shown below.
1.      IEC 61000-4-2, Contact Discharge ±30 kV
2.      IEC 61000-4-2, Air-Gap Discharge ±30 kV
3.      IEC 61000-4-2, Electrical Fast Transient ±4.4 kV
4.      IEC 61000-4-5, Lightning surge (8/20µs) ±30 A
AZRS3082 has complete System-Level ESD protection capability for pinA and pinB but also has embadeed whole-chip ESD protection circuit as shown in Fig.3. Not only the pinA and PinB gurantees to meet DL/T645 standard that sustain HBM (Human Body Model)15kV and the MM (Machine Model) protection is up to 800V for Industrial Networks application, but all of the other pins can sustain the HBM 15kV to fully protect the IC. For more reliable corner, the Charge Device Model (CDM) of the AZRS3082 can release more than 2kV charge, which is stored in the die and package. Moreover, the latchup immunity of the Amazing RS485 is up to 400mA,it is very suitable for noisy environment application.


                                                                                               Fig.2 Bi-direction TLP Curve of AZRS3082 with On-chip ESD/Surge TVS Embedded.
                                                                                             Fig.2 Lightning Surge Characteristics Comparison Chart of AZRS3082 and Competitor.
                                                                                                      Fig3. AZRS3082 WholeChip ESD Protection Circuit
AZRS3082 is Fully Compliant with the EIA/TIA-485 Standard
The AZRS3082 has a slew-rate-limited transmitter to minimize EMI and reflection caused by improperly cable terminated. The transmission speed of AZRS3082 is up to 250kbps, which is related to the twisted cable length. The longer cable cause larger signal dissipation to limit the transmission speed. The TX of AZRS3082 provides higher differential output voltage (VOD=VA-VB=2.1V) for cable loss. The input threshold voltage of the receiver from -50mV to -200mV to meet the EIA/TIA-485 standard and features a fail-safe receiver, which guarantees the output of receiver to logic high when the receiver inputs are open, short or idle.
For the glitch free protection, hot-swap circuit of the AZRS3082 guarantees the outputs of both the transmitter and the receiver in a high impedance state during the power up period. Moreover, the transmitter of the AZRS3082 has the thermal shutdown and the current limited function to protect itself from damage of the system fault conditions during normal operation.
AZRS3082 is Suitable for Low Power Application
AZRS3082 has energy saving feature for smart meter utility. In standby mode, AZRS3082 has ultra low power consumption which the leakage current is less than 1nA, which is suitable for the application for battery and remote control. Moreover, the short current prevent mode is to remove the short current in power up period and increase the power efficient to save more energy.
Amazing rely on ESD technology to develop the On-chip ESD/Surge TVS on traditional CMOS process to against the System-Level ESD and lightning surge issue. So, the AZRS3082 transceiver is embedded the robust On-chip ESD/Surge TVS inside. It has ultra low standby current and low quiescent current to meet longer standby time. Furthermore, the transmitter has larger differential output voltage margin for cable dissipation to provide longer cable transmission capability at the same data baud rate.
AZRS3082 is high performance and high reliable RS485 transceiver。


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