Electronic Design

PoE Chip Mixes Logic And Power Devices To Kickstart New Applications

The relative dearth of recent power-over-Ethernet (PoE) product announcements made me think that 802.3af had hit some kind of plateau. Perhaps people are waiting for the IEEE 802.3af subcommittee to wrap up its classification issues and make some kind of general pronouncement about how the extra power in PoE-Plus would be delivered (see "Controller Anticipates PoE Plus," May 25, p. 26).

Not quite. Silicon Laboratories has leapfrogged the competition in basic PoE powered-device (PD) chips. Its Si34000

controller integrates all the high-voltage devices (diode bridges, transient surge suppressor, and switching FET) on the same die as the logic.

This may encourage more OEMs to create a new generation of lower-cost 802.3af-compliant PDs, since the lower external parts count can be expected to cut assembly and bill-of-materials (BOM) costs. Silicon Labs reckons its Si3400 reduces external parts by 12 to 25 and costs by around a buck and a half. Integrating the power semiconductors with the logic has some other advantages, too.

For example, the Silicon Labs engineers figured that as long as they had the internal diode bridges connected directly to the RJ-45 Ethernet jack, they could implement a power-loss indicator circuit that uses the PoE classification

interface to alert the PD of any power loss in time for it to shut down gracefully. Direct connection to the jack also allows shorter trace lengths, which can help reduce electromagnetic interference.

The basic switching regulator runs at 280 kHz and can handle isolated as well as non-isolated loads. The switching FET handles up to 4 A. On-resistance is 300 m½. The bridges can handle 6-A transients. And, the integrated surge suppressor fires at 64 V.

The Si3400 comes in a 5- by 5-mm, 20-pin quad-flat no-lead (QFN) package. The part is sampling now, with volume production set for the third quarter of this year. Pricing begins at $2.48 in lots of 10,000.

Silicon Laboratories

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