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  • Si3462 Single-Port Power Controller for PoE Devices

  • Created: Feb 22, 2017

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The Ethernet is a link layer protocol of the TCP/IP which is composed of physical layer and the data link layer. Power-over-Ethernet is a technology that allows electrical current to pass through together with the data on a twisted pair of Ethernet cabling. The Si3462 is a single-port power management controller for IEEE 802.3at PoE/PoE+ compliant Power Sourcing Equipment (PSE). The IEEE 802.3 clause 33 is the standard for providing power to a remote Ethernet device on the same cable that is carrying data.

The Si3462 operates directly from an isolated nominal input voltage of 50V for PoE or 54V isolated input for PoE+. The initialization of Si3462 will start during power up or by making pin 8 (RST) low followed by a transition of high signal. Upon reset (RST asserted low), the voltage at the STATUS pin (as determined by the DIP switch and the associated resistor network) is sensed to determine the operating mode of operation of the Si3462. The detection process begins immediately after initialization. During the detection phase, the PSE probes will determine if a 25kΩ is present within the range of 23.75kΩ and 26.25kΩ. A valid power device must present its signal within the desired range, and the Si3462 uses a robust 3-point detection sequence to avoid inadvertently powering and damaging a non-PoE-enabled device. The Si3462 can operate either endpoint or midspan mode. Detection phase is sequenced approximately every 360ms until RGOOD is sensed, indicating a valid PD has been detected. The STATUS LED (D2) is flashed at an approximate rate of 1.5Hz to indicate that the PSE is searching for a valid PD. After the detection, the Si3462 implements the classification phase, which executes both the one-event and two-event physical layer classification methods. For one-event classification, the pass FET Q4 is turned on and programmed for an output voltage of 18V with a current limit of 75mA for 30ms. For the two-event classification, the 18V pulse is output twice with an 8.5V amplitude mark pulse for 10ms between the two classification pulses. If the class level of the PD is not within the supported level as set by the initial voltage on the Si3462's STATUS pin, an error is declared, and the LED blinks rapidly at 10Hz for two seconds before the Si3462 goes back to the detection cycle.

The Si3462 can be applied also in FTTH media converters, cable modem and DSL gateways and set-top boxes.