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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 

The automotive industry was once behind in electronics adoption, but today the automobile is at the forefront of the electronics technology curve. The newest cars coming off the assembly line are not only pushing the boundaries of aerodynamics and fuel efficiency, but telematics, infotainment and cloud connectivity is revolutionizing the driving experience. From multimedia consoles decked with high-speed data ports to camera assisted driving systems to a plethora of GPS antennas, the vehicles speeding along today's highway are converging closer to what once was only a futuristic vision.

While the marriage of the automobile and consumer electronics improves both the safety and comfort of the driving experience, the rise in electronic sub-systems also creates new EMC compliance challenges. Today's high-speed automotive signal interfaces—USB, HDMI, LVDS, Ethernet—are driven by transceiver ICs every bit as sophisticated as the silicon enabling the newest consumer gadgets.

These high-performance transceivers are fabricated on 65nm transistor geometries and below. As silicon geometries shrink with each revision of the IC, oxide layers become thinner and ESD becomes more problematic. Previous generations of transceivers with a liberal silicon I/O pad area afforded the chip designer an abundance of chip real estate. As such, system-level circuit protection knowledge was not as critical – the chipmaker had implemented robust protection clamps at the I/O of the transceiver. That's no longer true of today's high-speed integrated circuits.

Today's EMC engineer, living in the sub-65nm world, knows that relying on the on-chip ESD structures to provide a robust system protection level is a luxury of the past. To safeguard today's high-speed ICs, a good EMC strategy is essential. This most certainly incorporates a generous use of "off-chip" (external) ESD protection components – especially in the automotive industry where the expectations for quality and reliability are paramount. If a consumer damages his or her cable modem router – no problem – they can likely take the box back to the retailer and replace it with a new router. But, what happens if they damage the USB port on their new luxury SUV's display console? Neither the auto manufacturer nor the car owner is eager to support such costly replacements. As such, the expectations for quality are higher which means the EMC requirements are more stringent.

For ESD, at a minimum, automotive suppliers and OEMs require level 4 ESD immunity according to the IEC 61000-4-2 standard (±8kV contact discharge, ±15kV air discharge). ESD testing is applied to all the vulnerable data ports including keypads, antennas, LCD displays, camera connectors, data port connectors and bus nodes. However, many carmakers go beyond level 4 of the IEC 61000-4-2 model and impose their own specific ESD requirements. For instance, some manufactures require ESD testing at ±25kV air discharge and ±15kV contact discharge. They may even call out immunity to the ISO10605:2008 ESD standard. In this case, the passive component network is slightly modified, but the voltage threshold is charged higher – up to ±25kV.

TVS diodes
With low clamping voltages and sub-nanosecond response times, TVS diodes are ideal for safeguarding data ports from fast rise-time transient threats (ESD). To minimise the stress voltage to the transceiver IC, a protection device must reduce (clamp) the incoming ESD spike to a safe voltage threshold for the protected IC. During the ESD event, the TVS junction avalanches to provide a shunt path for the transient current while clamping the transient voltage. The clamping voltage is the maximum voltage differential across the TVS device during the transient (ESD) event. This voltage should be well below the destructive threshold of the transceiver IC.

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