On-chip integrated TVS device protects bus interface chip

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On-chip integrated TVS device protects bus interface chip [Copy link]

Introduction:

The Internet of Things will lead to a sudden increase in data communication volume, and the harsh and noisy environment of outdoor networking has put forward higher requirements for the reliability of data transmission. Transient overvoltage caused by static electricity will affect the correctness of data transmission and even damage the bus data transceiver chip. This article takes the RS485 bus interface chip as an example to describe the method of protecting the bus interface chip with built-in TVS devices.

text:

The concept of the Internet of Things, which was proposed during the 2008 financial crisis, is expected to be widely popularized within 10 years, and this technology will develop into a high-tech market worth trillions of dollars. The Internet of Things will lead to a sharp increase in data communication volume, and the harsh and noisy environment of outdoor networking has put forward higher requirements for the reliability of data transmission. Transient overvoltage caused by static electricity will affect the correctness of data transmission and even damage the bus data transceiver chip.

The long-distance communication network built based on RS485 protocol of power carrier communication meter reading system is generally set up outdoors. One bus connects dozens to hundreds of RS485 transceivers. Surge current generated by lightning often damages hundreds of RS485 transceivers online. As shown in Figure 1, it is a surge protection scheme for RS485 data bus. Gas discharge tube GDT is used as the first-level lightning surge protection, self-recovery fuse PPTC is used for surge limiting, and TVS is used for secondary voltage limiting. Among them, TVS1 is used for differential mode protection, and TVS2 and 3 are used for common mode protection, so as to achieve the purpose of protecting RS485 chip. When laying out the circuit board, it is required that TVS and the back-end circuit RS485 chip and ground are as close as possible to avoid chip damage caused by insufficient discharge speed.

Figure 1 Surge protection scheme for RS485 data bus

Adding discrete TVS devices will, firstly, increase the size of the electronic system and the system cost. Secondly, even if the nominal protection level of the device itself is high enough, if the placement and routing of the TVS on the printed circuit board are not carefully considered, the chip may be damaged due to untimely discharge caused by parasitics on the board. Thirdly, the external TVS device needs to be designed in conjunction with the on-chip ESD. If the on-chip ESD cannot effectively discharge the residual pressure, the system reliability will also be reduced.

Hunan Jingxin Microelectronics Technology Co., Ltd. has developed an on-chip integrated TVS device structure based on the 15kV contact discharge protection requirements of the RS485 interface chip bus port, and realized the on-chip integration of this device in the RS485 interface chip. The built-in TVS device avoids parasitics on the printed circuit board. At the same time, the TVS device is customized with ESD according to the bus interface requirements, which effectively improves system reliability.

As shown in Figure 2, the electrostatic discharge path of the RS485 chip bus interface pins. The design needs to consider: (1) the ESD protection network of the entire chip; (2) the coordination between the TVS device and the ESD window of the driver output tube; (3) the structure of the TVS device that discharges most of the current energy; (4) the process compatibility of the TVS device and the core circuit; (5) the layout of the output stage.

Figure 2 Electrostatic discharge path of bus interface

As shown in Figure 3, the built-in TVS device and its TLP test curve suitable for the RS485 chip bus interface developed by our company are shown in Figure 4, and the integration of the TVS device in the chip is shown in Figure 4.

Figure 3: Built-in TVS device suitable for bus interface chip and its TLP test curve

Figure 4 TVS device integration in chip

Hunan Jingxin Microelectronics Technology Co., Ltd. has accumulated five years of experience in the professional design of ESD/TVS devices and successfully developed built-in TVS devices suitable for interface chips, meeting IEC61000-4-2Standard contact discharge 15kV, higher than the industry's conventional IEC61000-4-2level4 8kV. Our company has experience in designing and developing TVS devices built into bus interface chips. Directly selecting our ESD/TVS devices or entrusting customized development can effectively improve the electrostatic reliability of customers' data transmission systems.