Power Electronics

Data Points

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PoE Controller Integrates PD Designs

Silicon Laboratories' Si3400 is described as the industry's most highly integrated, IEEE-802.3af-compliant, power over Ethernet (PoE) controller for powered device (PD) applications. Housed in a 5-mm × 5-mm, 20-pin QFN, this controller includes a complete PD interface with programmable classification and detection signature circuitry, diode bridges, a transient surge suppressor, a switching regulator controller and FET. The chip also integrates a dual current-limited hot-swap switch and protection circuitry such as thermal shutdown.

According to the company, the Si3400 eliminates up to 25 external components compared to competing solutions, reducing the pc-board footprint by 30% to 50% and the total bill of materials cost by as much as $1.50. The device targets applications such as wireless access points, voice over IP phones, RFID tag readers, point-of-sale terminals, security systems and cameras.

The Si3400 is fabricated in a BCD process with silicon-on-insulator (SOI) technology. Because of its high level of integration and architecture, the chip is able to offer some unique features. For example, with direct access to the line-side voltages from the Ethernet cable, the integrated diode bridges enable implementation of an early power-loss indicator. This function provides adequate time to save operating and status information before safely shutting down the PD.

The diode bridges also allow a direct connection to the RJ-45 connector, which minimizes the pc-board trace lengths between the connector and the Si3400. This helps limit radiated emissions, speeding certification required by PoE products. In addition, the integrated transient suppressor activates protection circuitry when high voltages are detected, improving overall device robustness and reliability.

The on-chip switching regulator supports both nonisolated and isolated applications. For example, the chip can implement a nonisolated buck converter using the on-chip FET to generate a 3-V output at more than 3 A. Or the regulator might be used to implement an isolated flyback converter. The Si3400 has been designed to operate seamlessly with both 802.3af-compliant power-sourcing equipment (PSE) and legacy PSEs that do not comply with the standard's inrush current limits.

Now sampling, the Si3400 is priced starting at $2.48 in quantities of 10,000. Full production is scheduled for the third quarter of 2006. An engineering evaluation board (Si3400-EVB) is also available now and will be followed by a complete Ethernet system evaluation kit in August.

Micro Thermoelectric Generator Targets ZigBee

The Thermo Life micro thermoelectric generator under development at Thermo Life Energy possesses the unique ability to develop small but usable power levels from temperature differences as low as 5°C. While the potential applications are virtually unlimited, Thermo Life Energy has been acquired by Applied Digital as part of a strategy to develop an energy-harvesting capability for wireless devices, particularly nodes based on the ZigBee specification, according to Marc Poulshock, president of Thermo Life Energy.

Dr. Ingo Stark, CTO of Thermo Life Energy and inventor of the Thermo Life low-power thermoelectric generator, explained that the device operates on the Seebeck effect used by other thermoelectric generators such as thermopiles to generate electricity from differences in temperature. However, the Thermo Life is implemented as 5074 thermocouple junctions connected electrically in series within a single disk-shaped package about half the diameter of a penny and roughly the same thickness. Working voltages can thus be developed at much lower temperature differences. The absolute temperature values are also lower, allowing operation between a 25°C source and a 30°C source, for example.

The Thevenin impedance of the latest Thermo Life unit is approximately 86 kΩ, and the device can drive a current of 36 µA at 3.1 V into a matching load when the difference between the applied temperatures is only 5°C. This results in 111 µW of electricity for that condition.

According to Stark, because the thermoelectric voltage for the device is proportional to the temperature difference, while the output resistance remains relatively constant for lower temperatures, the available power increases with the square of the temperature difference. However, the Thermo Life is not intended to serve as a primary power component that would compete with more mature products such as batteries or solar cells, though novelty applications such as a thermally powered wristwatch are certainly possible.

Rather, as stated by Poulshock, the main purpose for developing the Thermo Life is to provide energy harvesting for low-power applications where other technologies would prove inadequate or not possible. Because the ZigBee specification will require low power (operating up to several years on conventional batteries) and low cost (eventual BOM of $3 for total solution) by design, and is poised for widespread adaptation, it will be the most attractive immediate application for Thermo Life technology.

Delivery of the new Thermo Life device to advance customers is expected later this year or early in 2007. Manufacturing agreements for general mass production are in progress. The ZigBee specification is available for download under conditions of the user agreement at www.ZigBee.org. For more information about the Thermo Life and the physics behind it, visit Thermo Life Energy's website at www.poweredbythermolife.com.

MOSFETs Support sub-1-V Power Supplies

A family of depletion-mode MOSFET arrays from Advanced Linear Devices enables breakthrough precision in controlling gate threshold and subthreshold voltage characteristics in circuits with power supplies less than 1 V. Normally on without applied power, these devices are ideal for fail-safe circuits in alarms, battery backup circuits, energy harvesting, alternative energy and many other applications.

The ALD114804/ALD114804A and ALD114904/ALD114904A are n-channel matched-pair EPAD depletion-mode MOSFET arrays offered in quad or dual versions and in PDIP or SOIC packages. With applied power, these MOSFET arrays enable circuits to operate in the normally off mode with practically zero power consumption. They are well suited for many secondary power backup systems that must function without drawing power from the secondary power source, and they serve as low-power replacements for form-C electromechanical relays (see the figure).

Matched at the factory using the company's EPAD CMOS technology, ALD's depletion-mode MOSFET arrays offer an industry-low threshold voltage of -0.40 V ±0.020 V for the A-suffix grade or -0.4 V ±0.04 V for the standard grade. This precision threshold voltage enables circuits to run below 0.7-V threshold barriers. These devices also offer tight offset voltage matching and control. The arrays are well suited for a broad range of analog applications such as current sources, differential amplifier input stages, transmission gates and multiplexer applications typically used in RF, oscillator and nanopower circuits.

“As the industry drives toward secondary power sources, smaller batteries and longer operating life for a variety of end-user applications, these new EPAD depletion-mode MOSFET arrays enable circuit designers to maximize the usable voltage signal range to near rail-to-rail,” said John Skurla, marketing director of ALD. “The precision of these devices can jump-start a new wave of extremely low energy designs such as zero-power battery backup circuits and hybrid power harvesting circuits that enable dual-sourced power supplies.”

With zero applied input voltage (0 V at the gate terminal) the drain provides a sink current of 20 µA. These devices also could be used in the subthreshold mode and operated with nanoamperes of operating current.

The MOSFETs are available in sample and production quantities. Pricing starts at $0.91 in quantities of 1000, and all models are also available from Mouser Electronics (www.mouser.com). For more information, see www.ald.com.

Components Provide Single-Package Isolation of I2C Buses

An extension of the iCoupler portfolio of digital isolators, the ADuM125x family provides a single-package, hot-swappable solution for isolating I2C buses. By eliminating the optocouplers and support circuitry for I2C isolation, the ADuM125x devices reduce board space by as much as 80% versus existing designs, while lowering cost. Uses for the isolators include power-supply applications, and these components support both the SMBus and PMBus protocols.

Typically, I2C isolation has been achieved with optocouplers. Because optocouplers are inherently unidirectional, this approach required that the two bidirectional I2C channels be separated into two unidirectional channels. The resultant four channels were then each isolated with an optocoupler and recombined into two bidirectional channels.

Using the iCoupler technology, this same functionality can be implemented with two die in one package. One die contains multiple microtransformers that create the isolation barrier, and a second die houses the code and decode circuitry needed to convert between bidirectional and unidirectional signal paths. Both of these die are then co-packaged in an 8-pin SOP to provide the 2.5-kV isolation to both of the I2C lines. (A model with 5-kV isolation is also planned.)

Two versions of this device are offered. The ADuM1250 provides two bidirectional channels supporting a complete isolated I2C interface. The ADuM1251 provides one bi-directional channel and one unidirectional channel for those applications in which a bidirectional clock is not required. Both parts can operate up to 400 kbits/sec.

The ADuM1250 and ADuM1251 I2C digital isolators are sampling now with full production scheduled for October. In quantities of 1000, unit pricing is $3.38 for the ADuM1250 and $3.05 for the ADuM1251. For more information, see www.analog.com/icoupler.

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