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Electronic Design

PoE Controllers Manage Power Autonomously

The LTC4258/59 quad powered Ethernet controllers perform all of the power-management operations required to implement the IEEE 802.3af Power over Ethernet (PoE) standard in power sourcing equipment (PSE). Unlike some existing PoE controllers, these devices from Linear Technology can operate in a fully automatic mode without the intervention of a host controller. Another distinction is the ability to implement either ac or dc disconnect techniques as specified in the recently ratified IEEE standard.

The controllers perform powered-device (PD) detection and classification, −48-V power distribution, and fault protection independently on each of four channels. Housed in 36-pin SSOPs, the chips use an I2C serial interface to control application of power to Ethernet ports and to report port status back to the host system. With the ability to share the data bus among 16 controllers, up to 64 channels may be controlled using only two digital lines. An interrupt output can be used as an interrupt line to enable fault detection without software polling (see the figure).

In addition to its ability to operate autonomously, the LTC4258/59 can run in semi-automatic or processor-controlled modes. The devices require external power switches and current-sense resistors—-an approach that permits scaling of current and power dissipation levels and protects against overvoltage and overcurrent events.

Unit pricing in 1000-piece quantities is $6.35 for the LTC4258, which implements only the dc disconnect technique, and $6.95 for the LTC4259. Production quantities will be available in the fourth quarter. Linear also offers a demo board that provides a reference design for the 48-V power path portion of the PoE design.

The company has unveiled a PoE interface controller for PDs as well. The LTC4257 integrates almost all of the circuitry required by a powered device in an IEEE 802.3af-powered Ethernet network.

Linear Technology

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