2. System Overview

The following Block Diagrams will give the designer a sense for the internal arrangement of functional blocks, plus their relationships to

external pins. The Block Diagrams are followed by a description of the features of these integrated circuits.

2.1 Block Diagram

VPOS

VNEG

DETECTION

RDET

OSCs

fixed: 250kHz

adjustable: 100...500kHz

CLASS

PoE

CONTROLLER

HOT-SWAP

CONTROLLER

THERMAL

PROTECTION

HSO

5V

REGULATOR

RFREQ

VDD

RCL

CURRENT

MODE

PWM

CONTROLLER

THERMAL

PROTECTION

VSS

SWO

EROUT

FBL

FBH

TVS

100V

HSSW

250kHz

250kHz

VPOS-1.32V

VSS+1.32V

Start

ISNS

DC/DC

SW

VT15

AUX

WINDING

SUPPORT

Figure 2.1. Si3404 Block Diagram

2.2 Power over Ethernet (PoE) Line-Side Interface

The PoE line interface consists of external diode bridges, internal surge protection, and protocol interface support for detection and

classification.

The chip features active protection against surge transients and accidentally applied telephony voltages.

2.2.1 Surge Protection

ered). If the hotswap switch is on, the surge power is sunk in the dc-dc’s input capacitance.

The internal surge protection can be overridden with an external TVS if higher than specified surge conditions need be tolerated. The

external surge device must be connected in parallel to the internal one; therefore, the designer must ensure that the external surge

protection will activate prior to the internal surge protection.

2.2.2 Telephony Protection

The Si3404 provides protection against telephony ringing voltage. The telephony ringing is much longer than the surge pulse but it has

less energy, therefore, the Si3404 has a switch parallel with the supply (VPOS and VNEG). When the protection circuit is activated, it

turns ON the telephony switch; the ringing energy then dissipates on this switch and ringing generator resistance (> 400 Ω).

Si3404 Data Sheet

System Overview

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