Conformity Magazine - June 2008 - (Page 48)

100 ohms in the portions where there is no ESD device, and the device is designed so that it will meet signal integrity requirements without the need for any external compensation. This allows the same board to be manufactured at several PCB vendors without changing the layout, reducing design complexity, as well as development and bill of material costs. A side benefit of this approach is that it also provides improved ESD protection. To see why, it is useful to compare simplified circuit diagrams for the traditional ESD approach vs. the route-through approach. The internal circuit of a traditional ESD approach can be represented as shown in Figure 8. In this figure, the ESD device hangs off of the signal line. The parasitic capacitance of the ESD device creates the unwanted impedance drop in the line, while the bond wire inductance to the ESD circuit acts to slow down the ESD response time. Figure 9, in contrast, shows the ESD device in the “signal through” configuration, where L1 and L2 are integrated into the package to create a matched impedance with the outside line. As can be seen, the L2 inductor acts to slow down the rise time of the ESD pulse seen at the ASIC , resulting in lower peak voltages and currents than would be seen using a standard ESD diode. Lab results indicate that this approach results in a 40% reduction in peak voltage seen at the ASIC during an ESD strike, as well as a 15-40% reduction in peak residual current seen at the ASIC.4 Summary Higher speed circuits are increasingly prone to ESD damage as they migrate to smaller geometry silicon processes. Maintaining excellent signal integrity while providing adequate protection can be challenging, but there are several techniques available that can help the designer meet both goals: • When possible, choose impedance matched circuits that integrate inductance with capacitance to eliminate external impedance matching design challenges. With these techniques, it is indeed possible to achieve excellent signal integrity with excellent ESD protection. Jeffrey Dunnihoo is the senior systems engineer for PC and Consumer applications at California Micro Devices and can be reached at jeffd@cmd.com. Joe Salvador is the company’s Director of Marketing for Consumer and Computer products, and can be reached at joes@cmd.com. References 1. 2. 3. “Electrostatic Discharge Technology Roadmap”; ESD Association; March 4, 2005 “VESA DisplayPort Standard”; Version 1, Revision 1a, VESA, January 11, 2008 “Evaluating ESD Protection Circuits and Test Methods”, Joe Salvador, Conformity Volume 12, Issue 8, August 2007 Based on California Micro Devices’ test results comparing CM1233-08DE Picoguard XP vs. industry leading ESD TVS diode array. 4. FAST Link www.conformity.com/1860 • Choose low capacitance solutions, but make sure to compare ESD performance and use the I/O to ground junction capacitance; arrays rather than discretes to maximize capacitance matching and simplify layout; when laying out ESD components, taking into account all PCB board stackup information; Conformity JUnE • Choose integrated diode • Take care to match impedance Figure 9: Simplified circuit for “route through” ESD device 48 2008 http://www.conformity.com/1860

Table of Contents Feed for the Digital Edition of Conformity - June 2008

Conformity - June 2008 Contents Editor's Note 700 MHz Auction Raises a Record $20 Billion FCC Modifies PCS and AWS Power Limit Rules for Broadband Wireless Commission Designates Spectrum Test Bed FCC Releases Report on Wireless Competition Commission Proposes $5 Million Fine for Slamming Using EDX for Non-Destructive Detection of Lead in Consumer Products ESD Open Forum Challenges in Testing - Improving Election Security and Accuracy Part 1 Challenges in Testing - Human Body Model: The Hidden Challenges High-Speed Signal Integrity Considerations for ESD Components Focus On...EMC Components Buyer's Guide FCC Levies $2.6 Million Fine for Junk Faxes FDA Updates Guidance on its Product Review Process EU Issues Updated Energy Star Regulations EU Repeals Directive on Veterinary Electro-Medical Equipment New Product Announcements Updated Standards List for the EU's PPE Directive CPSC Announces New Effort to Keep Out Hazardous Products CPSC Actions in the News IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity - June 2008

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