Users are putting the squeeze on smaller yet more capable telecom and datacom systems for greater performance in electrically harsher environments, forcing overvoltage and overcurrent devices for protecting these systems into a corner. Indeed, it's a matter of tiny packaging vs. big raw-performance issues, a timeless challenge most recently addressed in the power sector with application-specific MOSFETs for switching supplies.
Finer-line IC traces, which make modern devices more highly susceptible to transients, have helped bring on the circuit-protection challenge. So have higher power densities and a host of related issues involving system functionality vs. the size of the system's enclosure. Thus, there's continuing emphasis on integrating both overvoltage- and overcurrent-protection devices on the same chip. Ultimately, it's leading to smaller and even chip-scale packages to minimize real estate.
Today's tighter layouts bring a growing need to address the problem of electrical disturbances as a single general entity. With packaging such an issue, two distinct areas-circuit protection and electromagnetic/radio frequency solutions-may merge more fully in future designs.
Traditional silicon-based transient-voltage suppressors (TVSes, essentially fast p-n junction diodes that serve to clamp voltage to the lowest practical level for a given application) and metal oxide varistors (MOVs, voltage-sensitive resistors with generally higher clamping voltages) still rule the roost for overvoltage and electrostatic-discharge (ESD) protection. These handle a broad spectrum of datacom and telecom applications, desktop through portable.
In general, the traditional silicon device finds applications in multiline devices at data rates to a few hundred megabits per second. The varistors, meanwhile, are most often used in low-frequency, bypass or single-line applications.
In a snapshot of the activity in the overvoltage area, AVX (Myrtle Beach, S.C.), which characterizes some of its devices for collective use in circuit protection and EMI/RFI applications, has tucked its MultiGuard series of bidirectional TVSes in smaller package sizes. For ESD applications as well as EMI attenuation, they're now available in 0405 two-element, 0508 two- and four-element, and 0612 four-element case sizes at standard working voltages of 5.6, 9, 14 and 18 V, with energy ratings of 0.1 joule. A StaticGuard version of these TVS arrays is also available with a total capacitance of less than 75 pF. The arrays take up less than half the printed-circuit board real estate required for the equivalent number of discrete chips.
Other suppliers include Semtech (Newbury Park, Calif.), mainly a TVS maker that largely uses a diode-array technology and various proprietary IC techniques, including an enhanced punch-through technology, depending on the application.
Semtech has at least nine application-specific series to address control lines, high-speed lines, high-speed and low-voltage lines, and telecom and lightning protection. Its latest devices include the SRV05-4 four-channel TVS array for Ethernet, notebook, handheld and PDA applications. Part of the company's RailClamp family, it has a typical capacitance of 3 pF per line, which Semtech touts as less than half the capacitance of competing products for communications applications to 1 GHz.
The SRV05-4 is billed for protecting sub-5-V CMOS line transceivers from transients caused by ESD (protection greater than 15 kV), lightning and other induced voltage surges.
"Until now, designers have been forced to use inadequate protection schemes due to capacitance constraints," said Tom Dugan, Semtech's director of marketing for protection products. "This array makes protection of high-speed interfaces possible and affordable to engineers working on state-of-the-art telecom and datacom designs. In fact, we have one customer successfully employing the device on lines running as high as 1.25 GHz."
Earlier, the company released the LC03-3.3 device for lightning, ESD and electrical fast-transient events, terming it the industry's only low-cap surge-protection device that complies with Bellcore, ITU and IEC standards for 10/100 Ethernet lines. Also, Semtech's LC05-6 is intended to provide secondary surge protection for two signal pairs in telecom and datacom systems, meeting Bellcore's GR1089 intrabuilding standard as well as ITU K.20 and IEC 61000-4-5.
Generally, chip-scale solutions are becoming a bigger part of the overall packaging landscape. Late last year, Semtech opened the way with a TVS in its TransClamp family, the TC03A-200 for tip- and ring-line pairs, which in SOT-89 was billed as 15 percent smaller than competing packages. About the same time, the company introduced the SMF05C multiline TVS in an SC-70, and then a four-line TVS array in chip-scale for cell phones, PDAs and pagers. This device, the SFC05-4, takes up only 1.5 square millimeters of board space and is less than 0.65 mm high.
Among the new devices from Littelfuse (Des Plaines, Ill.) is the company's first 0402-size (1 x 0.5 mm) chip-scale avalanche-diode-based TVS for ESD suppression in analog and digital signal lines. The SP0501BAC, with a capacitance of 30 pF, is rated for an ESD of 30 kV, low clamping voltage (13 V), subnanosecond response time and low leakage current.
Littelfuse also recently released one of the few new MOV devices, this one with a twist: Its iTMOV integrates a thermal sensor and includes a third terminal that indicates when the thermal element has opened the circuit due to overheating caused by an overvoltage condition.
AVX is another company specializing in MOVs, with the TransGuard series of low-capacitance multilayer ceramic devices useful for infrared and optical applications.
For the highest-speed applications requiring overvoltage protection, polymer-based ESD components are coming on the scene. These are essentially metal-doped plastic devices that take on the characteristics of a varistor. Device makers characterize polymers as having the inherently lower capacitance (frequently below a picofarad) that is so critical to operation in applications where data rates are high.
Polymers also tout much lower leakage currents than silicon (fractions of a nanoamp), and comparable cost.
The specialists include Littelfuse, one of the few companies that also carries traditional metal-fuse technology, which still enjoys wide use but has largely made way for positive-temperature-coefficient (PTC) devices for overcurrent applications. Over the last six months, Littelfuse has taken its PulseGuard polymer to the next level with the ability to fabricate overvoltage devices on board during the board-making process.
Other entries in the polymer ESD arena come from Cooper Electronic Technologies (Boynton Beach, Fla.), which recently released its 41206 four-element-array chip. These bidirectional 12-V surface-mount devices, part of the company's SurgX line, clamp transient voltages typically to 35 V (maximum of 60 V).
In other work, Bourns Inc. (Riverside, Calif.) has introduced yet another voltage-protection device-a thyristor (silicon-controlled rectifier). The company's Tisp line is specifically built for lightning strikes and power-line crosses for telecom. It's designed to be used with Bourns' multifuse line of PTC devices for overcurrent protection and its Line-Feed surge resistor networks to provide comprehensive protection.
In addition, Bourns has just announced the 4B04B line of protection network products. They're also intended to be used in conjunction with the Tisp line to provide both overvoltage and overcurrent protection for telecom line cards, and secondary circuit protection in central offices and remote equipment.
When it comes to overcurrent protection, the acknowledged leaders are PTC devices, largely achieved by polymers that essentially behave as a thermistor. This technology is different from the polymer-based technology used for ESD devices.
Last year, Raychem Circuit Protection (Menlo, Park, Calif.) introduced the nanoSMD series as the smallest surface-mount devices in the company's polymeric positive-temperature-coefficient line. Raychem also extended its Polyswitch products for actuator and sensor applications with the RXE005.
The radial-leaded product is said to have a low (50-mA) hold current; this tight tolerance allows it to protect closer to the actual operating current. With a trip current of 100 mA, the RXE005 is rated at 60 V and is capable of interrupting up to 40 A.
Earlier, the company addressed battery protection, releasing its VTP110 resettable strap-form device, touting the thinnest strap profile (2.9 mm) available for thin lithium-ion prismatic cells. At the same time, Raychem introduced its MAC300 polyswitch for the new AAAA nickel-metal-hydride cells, which provide increased capacity in less space for portable electronics.
Raychem also announced the BBR550 and BBR750, a new, coated version of the 90-V radial-leaded PolySwitch to limit current for cable telephony in the event of power-cross faults on a coaxial cable, such as might be generated by phase lags in a multiphase power-distribution system.
Other players include Vishay Ceramite (Grafton, Wis.), with its ceramic PTC thermistors-temperature-sensitive resistors built from barium titanate. The same material used in capacitors, it is made using some of the same basic processes. The PTC devices find use in motor starting, ballast starting and overcurrent protection. They are generally rated at high operating voltage and can withstand the continuous-switching characteristics of high-current applications.
The trend toward more highly packed pc boards includes the inevitable migration to surface-mount fuses. Raychem, for instance, introduced the nanoSMD Polyswitch surface-mount devices last year. Teccor Electronics (Irving, Texas) launched its TeleLink, which is intended for use with the company's thyristor TVS, the Sidactor, specifically for telecom. These fuses, touted for eliminating series line resistance and surge-rated to 100 and 500 A, are designed to carry 100 percent of rated current for up to four hours and 250 percent of rated current for 1 second minimum, 120 seconds maximum.
Another new entry, from Littelfuse, is the 1206L series surface-mount resettable PTCs, which are rated for 0.2- to 1.5-A service. The devices are one-third the size of the company's 1812L series, making them Littelfuse's smallest PTCs.
With circuit-board densities climbing, devices are destined to include a collection of overvoltage, overcurrent and even EMI/RFI elements on the same structure. This stems from the need to handle such multiple protection challenges as EMI, hot plugging and noise from lighting devices and displays.
STMicroelectronics (Lexington, Mass.) took the initiative in this realm, placing more overcurrent and overvoltage devices on the same chip, as seen in its bidirectional CLP30 and CLP270 lines for lightning protection in telecom applications.
Among the more recent releases is the T5 series for overvoltage and overcurrent protection for telecom from Teccor Electronics. This product is not a chip but a five-pin module for OEMs measuring about 2 x 0.5 x 0.75 inches, with the overvoltage version containing the company's Sidactor technology and the overvoltage-with-overcurrent version using a thyristor and PTC device. It's intended to protect high-speed digital subscriber lines.
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