DEMO MANUAL DC1145B LTC4267 Power over Ethernet Powered Device With 12V Isolated Supply DESCRIPTION Demonstration circuit 1145B, featuring the LTC 4267, rent limit allows the LTC4267 to charge load capacitors provides a complete IEEE 802.3af Powered Device (PD) and interface with legacy PoE systems. interface and isolated 12V at 0.9A power supply solution The LTC4267 combines the above features with a current for use in Power over Ethernet (PoE) applications. An mode switching controller designed for driving a 6V rated optional connection is included for a 12V auxiliary supply N-channel MOSFET. It features programmable slope com- on the secondary side that disables PoE when present. pensation, soft-start, and constant frequency operation, The LTC4267 integrates the 25k signature resistor, clas- minimizing electrical noise even with light loads. sification current source, thermal overload protection, Design files for this circuit board are available at signature disable and power good signal along with an DEMO MANUAL DC1145B OPERATING PRINCIPLES After detection and classification, the PD is powered up is achieved through transformer T1, capacitor C14 and when the input voltage exceeds the LTC4267 turn-on under- opto-isolators ISO1 and ISO2. voltage lock out (UVLO) through a dual-level current-limited The primary side power path is comprised of C1, L1, C9, power switch. The 5F capacitance minimum requirement of T1, Q2, and R6. These components should be as after the in-rush circuit is met with C1 and C9. While the close to each other as possible when laying out the printed voltage between P and VPORTN is above the Power OUT circuit board. In an actual implementation, a snubber Good trip point, the amperage through the power switch circuit can be added across Q2 to reduce voltage spikes is held below the low-level current limit. When the volt- seen at the FET. The secondary side power path is made age between P and VPORTN is below the Power Good OUT up of the other of T1, D1, C4, C5 and C6. These parts trip point, the Power Good signal goes active low and the should also be laid out as close to each other as possible, amperage through the power switch is held below the without overlapping any of the circuitry or traces of the high level current limit. primary side. For the PD to remain powered on, it must present to the When a 12V auxiliary supply is connected at connect J2, PSE both AC and DC components of the Maintain Power which takes over as the supply for the output. ISO2 provides Signature (MPS). The PD must hold the DC MPS by an isolated signal from the auxiliary supply to disable the drawing at least 10mA or the PSE may disconnect power. LTC4267. All LTC4267 signature detection, classification To meet the AC MPS, the PD must present 27k of AC and the internal power MOSFET switch are disabled. A PSE impedance or less. The LTC4267 will remain on with a PSE that uses DC disconnect will remove power. The LTC4267 that uses the AC disconnect method since it will always can resume normal PoE operation when the auxiliary meet the AC MPS. supply is removed. If power was removed, then the PSE The LTC4267 flyback converter operates at a typical will perform a new detection and then power the device. switching frequency of 200kHz, controlled by the current mode controller portion of the LT4267. Galvanic isolation dc1145bf 2