對接口進行ESD保護的實現方案

　　雖然片上ESD結構被用作極好的次要保護，但是它們通常對長期暴露在自由環(huán)境下的接口保護不足。

　　“模擬域”部分將焦點放在IC、子系統和系統設計者面對的不斷增長的電路設計挑戰(zhàn)：接口ESD保護。沿著工業(yè)現有技術的軌跡，接口保護難度按兩個趨勢日益增加。首先，增加了處理速度和功能密度的需求，推動IC制造者進一步縮減MOS裝置的最小尺寸。對給定的門絕緣體，設備原理上需要維持絕緣體厚度與橫向尺寸成比例（參考文獻1）。然而，較薄的絕緣體更容易受ESD的影響。

　　其次，端口速度持續(xù)增加，特別是便攜式多媒體設備的興起。例如，1996年發(fā)布的USB 1.0提供12Mbps的接口。四年后，USB 2.0達到480 Mbps。今年Intel開發(fā)者論壇出現，但仍未發(fā)布的USB 3.0，用平行光纖電纜替代銅連接器達到4.8Gbps的連接速度。IEEE 802.3（以太網）和IEEE 1394（火線）或多或少顯示了趨勢。

　　傳統上，包括ESD分流結構，帶標準互連焊盤單元的IC保護內部電路，避免ESD沖擊設備引腳。如果其結構連接嚴格的抗ESD處理程序，對制造商而言，片上ESD保護板裝配的IC安全是必要

的。雖然片上ESD結構被用做極好的次要保護，但是它們通常對長期暴露在自由環(huán)境下的接口保護不足。

　　這些結構不足的原因有兩部分：它們太小到不能吸收接口常出現的瞬態(tài)大能量，遠離接口入口點放置，防止鄰近軌跡的耦合。一旦I/O線的片上ESD單元開始工作，瞬態(tài)電流沿走線增加，引起走線感應，產生相應瞬態(tài)電壓，Ldi/dt。鄰近I/O走線的互感導體滿足超過其自身片上ESD單元容量的瞬態(tài)現象。這些鄰近軌跡包括時鐘、數據或其他非端口信號（參考文獻2）。

　　裝配TVS（瞬態(tài)電壓干擾抑制器）到與入口點盡可能近，通過瞬態(tài)電流進入產品前分流ESD到地，來減少問題。這樣做，用最小的附加費用，充分地增加了產品的魯棒性?！叭绻种附咏?jié)點，保護它”是一個好準則。盡管鍵帽是絕緣的，這個準則也應用到鍵區(qū)開關線。

　　高速接口需要帶特殊低旁路電容的TVS。檢查銷售商的產品線，特別是高速接口設備。如果不能找到設備的特定接口，與銷售商明確提供的標準接口比較信號頻率和源線特性?？紤]TVS制造商的要求，確保理解特定設備的ESD源模型。例如，JEDEC HBM（人體模型）通過1.5kΩ resistor，使用100pF電容放電。測試模型的可重復性結果歷史上就是關注區(qū)域（參考文獻3）。IEC-61000-4-2標準的測試方法提升了可重復性，其源模型——150Ω后的150pF——使設計要求更苛刻。

　　英文原文：

　　Protecting interfaces from ESD

　　Though on-chip-ESD structures serve as excellent secondary protection, they are usually insufficient to protect an interface over years of exposure in uncontrolled environments.

　　By Joshua Israelsohn, Contributing Technical Editor -- EDN, 12/3/2007

　　This installment of “Analog Domain” focuses on one of the growing circuit-implementation challenges that confronts IC, subsystem, and system designers alike: interface ESD protection. Two trends make interface protection increasingly difficult as we move along our industry’s current technology trajectory. First, increasing demands for processing speed and functional density have pushed IC fabricators to shrink further the minimum dimensions of MOS devices. For a given gate dielectric, device physics require maintaining the dielectric thickness in proportion to lateral dimensions (Reference 1). Thinner dielectrics, however, are more susceptible to ESD overstress.

　　Second, port speeds continue to increase, particularly with the growing success of portable-media devices. As one example, the 1996 USB 1.0 release provided for 12-Mbps interfaces. Four years later, a USB 2.0 feed could reach 480 Mbps. The yet-unreleased USB 3.0, shown at this year’s Intel Developer Forum, trades copper interconnect for parallel fiber-optic cable to attain a 4.8-Gbps connection. IEEE 802.3 (Ethernet) and IEEE 1394 (Firewire) show the trend to a greater or lesser degree.

　　Traditionally, ICs include ESD-shunting structures within their standard interconnect-pad cells, which protect the internal circuitry from ESD strikes to the device’s pins. The on-chip-ESD protection may be all that’s necessary to see an IC safely from its manufacturer to a board assembler if both organizations adhere to strict anti-ESD-handling procedures. Though on-chip-ESD structures serve as excellent secondary protection, however, they are usually insufficient to protect an interface over years of exposure in uncontrolled environments.

　　The reasons these structures are insufficient are twofold: They are too small to absorb large energy transients that interfaces commonly experience in the field, and they reside too far away from the interface-entry point to prevent coupling to adjacent traces. Once an I/O line’s on-chip-ESD cell begins to conduct, a current transient develops along its trace, exciting the trace’s inductance, causing a corresponding voltage transient, Ldi/dt. Adjacent conductors with mutual inductance to the I/O trace see coupled transients that can exceed the capabilities on their own on-chip-ESD cells. These adjacent traces can include clock, data, or other nonported signals (Reference 2).

　　Mounting TVSs (transient-voltage suppressors) as close to the entry point as possible alleviates this problem by shunting the ESD event to ground before transient currents develop inside your product. Doing so can add substantially to your product’s robustness with minimal additional cost. “If a finger can get close to a node, protect it,” is a good rule of … errrrr … thumb. This rule applies to keypad switch lines, too, despite their insulating keycaps.

　　High-speed interfaces require TVSs with particularly low shunt capacitances. Check your vendor’s product line for devices built for specific high-speed interfaces. If you don’t find devices specifically for your interface, compare your signalin

g frequencies and source-and-line characteristics with those of interface standards that the vendor explicitly supports. When considering TVS manufacturers’ claims, be sure to understand which ESD source model they use when specifying their devices. The JEDEC HBM (human-body model), for example, uses a 100-pF capacitor discharging through a 1.5-kΩ resistor. The repeatability of the test method’s results has been historically an area of concern (Reference 3). The IEC-61000-4-2 standard’s test method promotes repeatability, and its source model—150-pF behind 150Ω—is more demanding of your design.

　　References

　　Sicard, Etienne and Syed Mahfuzul Aziz, “Application note on 45-nm technology,” Institut National des Sciences Appliquées de Toulouse, June 27, 2007.

　　“Disadvantage of on-chip transient protection,” Application Note SI97-04, Semtech, September 2000.

　　Verhaege, Koen, et al, “Recommendations to further improvements of HBM ESD component level test specifications,” Proceedings of the Electrical Overstress/Electrostatic Discharge Symposium, September 1996.