Electronic Design

Components: Circuit Protection & Sensors

Circuit-Protection Components Scale Down Size, Speed Up Performance
Just like with other passives, the growth of portable applications has created a need for smaller circuit-protection components. Both overvoltage and overcurrent devices have been affected. With overvoltage, vendors are shrinking varistor and diode packages to achieve electrostatic discharge (ESD) and transient-voltage protection.

However, new technologies may be required to continue miniaturization. For example, ESD protection components are already available in case sizes as small as the 0402. Shrinking them further, while still making them robust enough to dissipate the high energy associated with an ESD event, will be a great challenge.

But fabricating smaller discretes is not the only solution to space problems.Vendors are also packaging arrays of these components in surface-mount and even chipscale BGA packages to reduce their overall board-space requirements.

Furthermore, packaging isn't the only concern for circuit-protection developers. As datacom protocols move to higher speeds, the capacitance of ESD protection components becomes a threat to signal integrity. In response to this problem, vendors are turning to polymer-based technologies that achieve ESD protection with ultra-low capacitance.

Further development of solid-state circuit breakers will be exploited to detect fluctuations in voltage and current as well as manage the interruption. For example, E-T-A (www.etacbe.com) plans to introduce an electronic circuit breaker for the 24-V output switched-mode power supplies used in process control. The circuit breaker will apply active current limiting to inrush currents, overloads, and shorts. The device will selectively disconnect only the faulty circuit, maintaining operation for the power supplies' other loads.

The integration of circuit-protection devices such as PPTCs with other active and passive components is coming, although probably not this year. For now the barrier is cost, but improvements in packaging techniques and increased demand for miniaturization could force this union of disparate technologies.

Among positive-temperature-coefficient (PTC) overcurrent protection devices, users can expect to see as much as a 50% reduction such as Raychem's (www.raychem.com) planned into of a 1-A device now offered in an 1812 case should be available in an 0806 case.

Higher-speed datacom protocols will foster furhter development of sub-pF ESD protection parts.

The 2002 EOSD/-ESD Symposium & Exhibits, Oct. 6-10, Charlotte, NC, will present the lastest research into electrical overstress and ESD.

Cost pressures in some applications may cause designers to reevaluate their application of circuit-protection components. Consider the cell-phone application where makers of Li-Ion battery packs have traditionally employed two levels of cell protection—an IC and a passive component—to prevent overcharging. Circuit-protection vendors are finding that some companies are taking calculated risks with their protection devices and eliminating one of the devices to reduce battery pack costs.

See associated timeline.

Integrated MEMS Sensors Branch Out To More Applications
Advanced sensor technology has become synonymous with microelectromechanical systems (MEMS). Certainly, "legacy" pressure, temperature, and motion sensors continue as the workhorses of the industry, filling the need for numerous specialized transduction functions. But micromachining technology, particularly MEMS, is rapidly overtaking their role and making many exciting, new applications feasible. Driven by a rising level of integration, MEMS sensors are now becoming commercially available for a wide range of applications, including biomedical, commercial, consumer, industrial, and automotive uses.

Manufacturers of digital ICs haven't particularly rushed to integrate more of the analog front- and back-end components that interface with the real world, like sensors and actuators. This may be due to their lack of analog design expertise. On the other hand, some analog sensor makers have taken the bull by its horns and integrated their devices with lots of digital circuitry. Many use innovative and promising solutions to meet market performance and cost demands.

To date, most of these efforts have concentrated on integrating the front-end analog transduction sources with digital signal-conditioning and processing circuits. Back-end actuation devices like switches made with microfabrication techniques certainly can't be too far away. MEMS actuators, such as switches, micromotors, and pumps, have already been demonstrated for a number of years. As for total front- and back-end integration, expect to see a complete closed-loop control circuit on one chip—that is, sensing, processing, and acting on the sensed information in one fell swoop.

Hardly an application will escape the influence of micromachined sensors and actuators. They can be made extremely small, so they dissipate very little power. They also are fully compatible with standard IC processes. This will expedite the development of what was before the missing link—the joining of the ubiquitous world of digital computing with the real world of analog functions.

Expect to see an implantable MEMS hearing device for the deaf, courtesy of the University of Michigan. It's under the direction of Ken Wise, a recognized MEMS pioneer, particularly in neural implants.

This year should bring the first commercial DNA lab-on-a-chip for the biomedical community. HandyLab (www.handylab.com), a spinoff from the University of Michigan (www.umich.edu), where the chip was developed, is focusing on making a handheld nanoliter-scale disposable microfluidic device for DNA diagnostics. It will supply physicians and health-care providers with real-time information to make on-the-spot diagnostics.

Implantable micromachined monitoring and drug-delivery systems, complete with sensors, actuators, and fluidic channels, are on the way. These will aid in the treatment of cranial and neural disabilities, as well as diabetes and coronary diseases. Already, batteryless implantable medical pressure sensors for monitoring brain activity are available from ISSYS (www.mems-issys.com). They use RF to transmit brain data to a readout.

Watch for monolithic MEMS ICs with sensing, control, and actuation on one chip. Italy's STMicroelectronics (www.stmicroelectronics.com) has used its closed-loop MEMS controller, the L6671 announced last August, for a tenfold improvement in computer disk-drive densities.

See associated timeline.

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