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ESD protection for automotive electronics

Posted: 18 Feb 2013     Print Version  Bookmark and Share

Keywords:TVS diodes  Twisted pair ethernet  Signal integrity verification 

While safeguarding the interface transceiver is the most critical part of the protection design, it satisfies only half of the design criteria. Additionally, the protection component must not adversely impact the signal quality. Thus, an effective protection solution should present sufficiently low leakage current and capacitance such that the TVS component is "transparent" to the transmission line during normal data transmission. To achieve this, the capacitance of the TVS should be low enough for the bandwidth of the signal port it is protecting. As an example, for USB 2.0 the TVS junction should generally present less than 3pF. For a higher speed interface, like HDMI, the capacitance should be much lower – less than 0.5pF.

Twisted pair ethernet
As the vehicle is rapidly evolving, the trend in higher speed data interfaces will continue. One trend that is changing In-vehicle networking is the proliferation of Ethernet as a networking standard. To enable driver assistance cameras and even multimedia connectivity, the cars of tomorrow will deploy 100Mbps Ethernet with unshielded cables. As such, the Ethernet bus brings the advantage of reduced cabling costs and is better suited for high-bandwidth applications over conventional busses such LIN, CAN and Flexray. This trend means that the newer Ethernet interfaces will need robust, low-clamping transient protection solutions to adequately safeguard the newest Ethernet silicon. It is also important that these TVS devices maintain a low line-to-line capacitance so as to have minimal impact on the signal integrity. The figure shows an example of protecting a single-port BroadR-Reach Automotive Ethernet PHY device. This protection scheme uses Semtech's RClamp 0582BQ protection array in a line-to-line (bidirectional, low-capacitance) configuration.

Figure: An example of protecting a single-port BroadR-Reach Automotive Ethernet PHY device.

Signal integrity verification
To ensure signal integrity, design engineers can take additional steps. First, when available, choosing TVS components with flow-through packages can optimise trace routing. The flow-through layout allows the high-speed traces to run on one signal layer minimising excessive trace bends and eliminating vias. EMC engineers can also verify signal integrity utilising the standardised eye masks for the signal interface. Any adverse effect to the signal integrity will manifest in an eye diagram that encroaches in on the test mask. Conversely, a nice open eye (below) around the pattern mask shows a clean signal that complies with to the data ports standard.

As the electronic content in automobiles rapidly evolves, today's vehicles are connecting drivers to technology in a way that was previously unknown to the automotive industry. This growth in electronic sub-systems introduces new and unique ESD immunity challenges for automotive EMC engineers. With careful design and forethought into a comprehensive ESD immunity plan, the vulnerable data ports can be effectively safeguarded to meet even the most stringent EMC immunity and signal integrity requirement of today's high-speed data interfaces.

About the author
Timothy Puls is a product marketing engineer with Semtech Corp. and is responsible for transient voltage protection solutions for the communications infrastructure. He holds a bachelor's degree in electrical engineering from Texas A&M University.

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