With the rapid progress of 5G, big data and AI and continuous application of new technologies, the importance of intelligent technology in various fields has gradually increased, especially in automotive field. As a product closely related to everyone, automobiles will become more intelligent under the development of new technology and even change our outing lives.
One of the most important components of a smart car is ADAS (Advanced Driver Assistance Systems). Camera and center console display are the hardware foundation of ADAS. With the 360-degrees view and popularity of HD central console instrument in vehicle, there are higher requirements for transmission speed and stability of video data. FPD-Link and GMSL transmission buses are the most suitable solutions currently. The architecture of these transmission buses is similar, taking FDP-Link of TI as an example; the ESD protection solution is also applicable to GMSL bus of Maxim Integrated.
FPD-Link is introduced by TI. The first generation uses a single-pair twisted wire to transmit video data at a rate of 350M bps based on LVDS standard. The following FPD-Link II changes its standard from LVDS to CML (Current mode Logic) which uses only 1 differential pair to achieve transmission speed of 1.8G bps. At present, the latest FPD-Link III is able to transmit video and control signals with 4G bps highest transmission speed on differential pair and FPD-Link III supports POC (Power-over-Coax). While transmitting high-speed video data and control signal, it also transmits power which requires very high requirements for stability of the system during operation. It is speculated that FPD-Link IV is under development, and the transmission rate may be further improved. ESD and EMC will face more severe challenges.
FPD-Link III supports two modes: ①single-end system mode and ② differential system mode.
The single-end system mode is composed by a coaxial cable, on which 12V power can be transmitted. The maximum signal transmission speed is up to 4G bps. This mode integrates video data, control signal and power on a very long cable which reduces cable cost significantly and is meanly used on on-board cameras.
Differential system mode consists of two pairs of differential lines and is used for transmission of HD video. Because of high transmission speed requirement, the cable will not be very long. It is mainly used for HD center console display in vehicle.
Figure 1: FPD-Ling III architecture overview
Both single-end system mode and differential system mode need to transmit high speed signal. Under the complicated electromagnetic environment in vehicle, it is very important to prevent data transmitted from interference and to maintain stability of power. As stability of on-board system is getting higher, the requirements for ESD and EOS protection have become stricter. The international standards applicable to ESD and EOS protection of on-board system are ISO10605, ISO7637 and ISO16750.
Table 1: ISO standards and test mode.
Different from the ESD test standard IEC61000-4-2 for consumer products, the ISO10605 standard includes 150pF/330Ω, 150pF/2000Ω, 330pF/330Ω, and 330pF/2000Ω test modes, therefore the test environment will be more complicated. As for the protective component TVS, AEC-Q101 certification is also required to meet the high reliability requirements of on-board systems. AMAZING Microelectronic Corp. on-board TVS can provide excellent protection performance and has been certified by AEC-Q101 standard, we have been paying attention to customer needs and constantly launching new products.
For ESD protection of FPD-Link III single-end system mode, recommended TVS parameters should comply with Cj ≤ 0.5pF, Vbr (Breakdown Voltage) ≥ 13.2V, AMAZING AZ9817-01F, Cj=0.4pF(Typ.) will not affect transmission of high speed signal which indicates that the system maintains good stability during ESD test (clamping voltage curve is shown in figure 2). At the same time, the package of AZ9817-01F is only DFN1006 (0402), which can save PCB area. At present, some customers demand higher DC voltage resistance standards. For such application, it is recommended to refer to the standards of Cj ≤ 0.5pF and Vbr ≥ 1.1*VDC when selecting TVS specifications.
Figure 2: AZ9817-01F ESD clamping voltage curve (Tested by TLP)
For FPD-Link III applications in differential system mode, ultra-low capacitance TVS must be used. For such application, an integrated TVS such as AZ9143-04F is recommended. The package is DFN2510 which supports crossover layout and is more suitable for wiring of differential pairs. It should be noted that the AZ9143-04F should be placed after AC isolation capacitor. The TVS Cj = 0.45pF(Typ.). Figure 3 shows the result of eye diagram test using USB3.1 10G-bps. It will hardly affect the signal transmission of FPD-Link III, and the ESD clamping effect is excellent (as shown in Figure 4). Even if there is strong ESD energy from the outside, it can still protect the back-end chip from damage. This has been verified in a domestic Top Panel customer product. Recently, some customers have proposed to place the TVS directly on the connector side. The recommended TVS specifications refer to the standard of Cj ≤ 0.5pF, Vbr ≥ 1.1*VDC.