Conformity Magazine - June 2008 - (Page 47)

pad footprints (Co becomes smaller per unit area), but won’t completely offset the capacitance of the ESD device. Increasing Trace Inductance This is often called trace necking and can be very effective when only small amounts of compensation are required. Reducing the width of the trace in the vicinity of the ESD device will increase the inductance, offsetting some of the capacitance of the ESD device. Like the previous technique, this also faces PCB limitations. For example, on thin dielectric boards, if the capacitance of the ESD device is too high, it can be difficult to achieve matched impedance by simply reducing the trace impedance, as this may require traces that are too thin to be manufactured reliably. Adding Inductance Such as a Common Mode Filter (CMF) Common mode filters are typically thought of as EMI suppression devices, but they can be used effectively to compensate for ESD parasitic capacitance as well as to compensate for some signal skew caused by mismatched devices between two lines. However, they also require valuable PCB board space, and are usually more expensive than the ESD devices themselves. In cost-conscious consumer electronics and computing markets, adding a CMF is typically only done as a last resort if the other techniques do not work because CMFs naturally add their own distortion and signal attenuation parasitics as well. To deal with these problems for the designer, a new class of components are coming to market which do not require impedance matching while still providing excellent ESD protection. Rather than focusing solely on lowering capacitance, these devices incorporate additional inductance inside of the ESD package to offset the parasitic capacitance of the ESD diode. To illustrate, first consider the traditional ESD layout depicted in Figure 6. The traces are routed directly under the package to maintain trace length matching and minimize turns. On each trace, one pin is connected to the ESD device, while the other pin is not connected. The impedance in the area of the diode is affected by the capacitance of the diode itself, the capacitance of both the ESD pin and the NC pin, and finally by the trace going underneath the package. In contrast, this new approach removes the trace from under the package by routing up and through the package. As seen in Figure 7, this eliminates the parasitics of the trace under the package, and allows the inclusion of inductors in series with the diodes to offset their capacitance and achieve matched impedance. The primary benefit of this approach is that it removes all need of the system designer to perform controlled impedance layouts that require custom modifications based on the board and ESD device parasitics. Instead, the designer can simply specify in their design that traces should be laid out for JUnE 2008 Conformity 47 Figure 8: Simplified circuit for traditional ESD protection solution http://www.kikusui.com http://www.kikusui.com

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.