January 2009 - Circuit Protection

High-Speed Standards

However, at higher data rates, using these traditional methods can cause circuit loading and signal distortion. They're not likely to be suitable for designs using high-speed standards such as DisplayPort, External SATA (eSATA), Low Voltage Differential Signaling (LVDS), IEEE-1394 FireWire, or IEEE-802.3 Gbit Ethernet.

Happily, today's ESD protection devices are, in many cases, actually tailored for high-speed applications. It's not uncommon to find ESD-protected USB (Universal Serial Bus) serial drivers, for example, or HDMI (High Definition Multimedia Interface) video port ICs with on-chip spike protection. Some include on-chip EMI (electromagnetic interference) suppression circuitry, too.

Safe Mating

The use of such ICs ensures that mating or un-mating separable interconnects won't cause inadvertent destruction of I/O chips. That can happen in static-laden field applications where users may not be aware of the need to discharge cables before making connections.

Discrete components or dedicated ESD ICs can make short work of spike protection, but at least one company went a step further to make it easier to integrate them. Spectrum Control, a company making EMI filters, ceramic capacitors, and  connectors, now offers RoHS-compliant (Restriction of Hazardous Substances) USB connectors that integrate EMI and ESD filters in standard USB packages. The company's Series USB-F and USB-E connectors meet USB 2.0 specs, yet act as drop-in replacements for unfiltered USB connectors, These interconnects also eliminate the need to re-design boards.

Spectrum's USB-F types exhibit dielectric voltages to 125 Vdc. The USB-E types have reverse standoff voltage specs of 5 V and reverse breakdown voltages of 6 V.

Semiconductor houses also offer dedicated bus devices that include built-in ESD suppression. Texas Instruments,for one, offers a number of HDMI receiver and transmitter chips with high voltage spike protection on-chip. TI's TPD12S520, for example, is an integrated differential-pair HDMI receiver that includes IEC Level-4 protection against contact discharge potentials as high as ±8 kV. If you're designing your own bus interfaces, ICs such as TI's SN74AVCA406E level translators, also include 8-kV clamps, and lend themselves to easy board layouts.

Regardless of this level of protection, these ICs are built to work in high-speed circuits where capacitive loading must be minimized. TI's TPD12S520 HDMI video port chip, for instance, exhibits only 0.8 pF of capacitance, which is just the ticket for HDMI 1.3, with its 340-MHz bandwidth and 10.2-Gbit/s data rates.

Chips like these also can include Ioff and back-drive protection, too (back-drive can occur when a path exists for current to flow through a diode array via a data line). That's a plus for data-cable connections where one side may be in a power-on mode while the other is in a power-down mode.

In such cases, current can flow back to a power-down circuit via the data cable. Where ICs provide this type of protection, no external current-blocking diodes need be specified.

Discrete Weapons

Of course, discrete ESD protection devices, and small arrays of diodes, are also weapons against ESD. TI's ESD protection devices, such as its TPDxE001 Series devices, are designed to protect I/O lines with rail-to-rail diodes. These arrays are compliant with IEC 61000-4-2, with up to ±15-kV protection. In a USB 2.0 480-Mbit/s application, for example, where plug-and-play operation is mandated, the USB spec calls for an input-capacitance of less than 5 pF in order to maintain signal integrity. The TPDxE001 Series meets the requirement, exhibiting just 1.5 pF of capacitance.

Vishay Intertechnology is another semiconductor house offering low-C protection arrays. Like TI’s devices, Vishay's latest 2-line and 4-line ESD arrays also exhibit very low capacitance, as well as low leakage-current specs. Vishay devices exhibit parasitic capacitance of 0.8 pF, with maximum leakage current specs of less than 0.1 µA (at the working voltage of 5 V). These arrays offer a typical breakthrough voltage of 7.9 V (at 1 mA), and a maximum clamping voltage of 15 V at a hefty 3.5 A.

Packaged in RoHS- and IEC-compliant ultra-low profile (0.6-mm Z-height) surface-mount packages. The 2-line VBUS052CD-FAH and 4-line VBUS054CD-FHI come in leadless packages. Like TI’s TPD12S520 chips mentioned earlier, these devices also feature flow-through terminal layouts that can ease PCB layout and save space in compact designs.

Vishay also offers two low-C quad ESD diode protection arrays in tiny molded packages. With 1 x 1-mm footprints and 0.4-mm profiles, Vishay's VESD05A4A-HS4 and VESD09A4A-HS4 diode protection arrays can save board space and provide 4-line ESD data port protection in space-limited nomadic designs. Both RoHS-compliant arrays provide transient protection for all four data lines as per IEC 61000-4-2 at ±15 kV for contact discharge and ±20 kV for air discharge.

Surge Protection Combos

Not all ESD protection applications are for mobile products. After all, ESD can be just as much of a problem for stationary products. Akros Silicon, for one, addresses this instance with active parts that combine system-level EMC compliance with surge protection.

Part of the company's AS1600 family of active EMI suppression devices, the latest AS1602 active device, packaged in a 16-pin QSOP, can be used for surge protection of Ethernet applications, for example, where greater than 10 dB of common-mode suppression is required. In a typical Ethernet application, an AS1602 is installed between the PHY connection and line transformer. It can be operated from 3.3 V or 2.5 V, dissipating about 90 mW.

The RoHS-compliant AS1602 offers ±25 kV of protection, without impacting high-speed signal integrity, thanks to a high differential-impedance and low capacitive loading. Akros' device can suppress common-mode noise in both directions, boosting a system's immunity to common-mode noise coming from an Ethernet cable, and suppressing emission of common-mode noise.

Melding Technologies

Emphasizing small size, STMicroelectronics' 9-cent EMIF01-1003M3 and 15-cent EMIF02-1003M6 circuit-protection devices come in 0.6-mm2 packages, which ST claims is the industry's smallest package that melds both EMI filtering and ESD protection. Slated for protecting keypads and connectors, these diminutive protection devices should fill the bill for nomadic phones, portable computers, peripherals, and the like, where real estate is paramount.

ST claims its 0.6-mm-high EMIF01-1003M3 can reduce board area requirements by roughly 55% for a typical 23-key keyboard application. The 2-channel EMIF02-1003M6 comes in a 1 x 1.45-mm micro-QFN 6-lead package that's about 45% smaller than some competitive devices that also combine protection and filtering.

For their part, the EMI filters meet IEC61000-4-2 Level 4 specs for ESD protection, and they exhibit a line-to-ground capacitance of 30 pF, which is low enough to preserve signal integrity in many moderate-speed applications. Both devices handle 15-kV air discharge and 8-kV contact discharge, in accordance with the IEC61000-4-2 spec. Clamping voltage is 9 V for the EMIF01 and 17 V for the EMIF02, when subjected to a 15-kV ESD strike.

Similarly priced devices, also in very small packages, are available from ON Semiconductor. Its products are based on a process that confers enhanced clamping with low capacitance and small die size. ON Semi's ESD7L5.0D, for example, is in a  1.2 x 1.2 x 0.5-mm SOT-723 package. It's priced at 20 cents a pop.

A single ESD7L5.0D can protect two data lines, exhibiting just 0.5 pF per line. Significantly, the 3-pin package gives you a way to protect the D and D- lines of a USB 2.0 port with a single device. An ESD7L5.0D could also be connected cathode-to-cathode to protect a single bi-directional line at 0.25 pF. The company suggests that could make it practical for protecting RF lines and possibly antenna ports.

ON Semiconductor's NUP4212 is available in a UDFN-6 package priced at about 60 cents in OEM quantities. Measuring a bit larger than a ESD7L5.0D, a NUP4212's 1.6 x 1.6 x 0.5-mm UDFN-6 package can protect four data lines, exhibiting 0.7 pF per line, as well as a pair of Vcc power lines. It also lets you protect D and D- and Vcc lines, but on two USB ports with a single device.

The ESD7L5.0D and NUP4212 are capable of nanosecond clamping an input ESD waveform of 15 kV to less than 7 V. ON Semiconductor claims competing low-capacitance off-chip ESD technologies, such as polymer and ceramic-based MOVs, offer low capacitance, but their ESD clamping voltage can be significantly higher. Moreover, ON Semi emphasizes that its devices are free from wear out mechanisms (MOVs are notorious for wear-out) that can affect reliability after multiple surge events.

So, there you have a quick overview. This article's scope and range is limited, but the information presented indicates how vendors are responding to your needs, where speed, space, and reliability are paramount considerations. To explore details about these product offerings, go to the Web sites of the mentioned vendors, but be sure to check out other manufacturers such as Bourns, OnChip Devices, Microsemi, NXP Semiconductors, and Semtech, to name a few.