LM5073

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SNVS490C – MARCH 2007 – REVISED APRIL 2013

Description of Operation and Applications Information

The LM5073 integrates a fully IEEE 802.3af compliant PD interface with versatile auxiliary power support. When

combined with a separate DC-DC converter, it provides a complete power solution for Powered Devices (PD)

that connect to PoE systems.

The LM5073 provides the following features:

1. The input voltage rating up to 100V allows greater flexibility when selecting a transient surge suppressor to

protect the PD from voltage transients encountered in PoE applications.

2. The integration of the PD signature resistor, inrush current limit, programmable input voltage under-voltage

lock-out (UVLO), PD classification, and thermal shutdown simplifies PD implementation.

3. The PD interface accepts power from auxiliary sources including AC adapters and solar cells in various

configurations over a wide range of input voltages. Auxiliary power input can be programmed to be either

non-dominant or dominant over PoE power.

4. Programmable DC current limit to support PD applications requiring input currents up to 800 mA.

5. Complementary open drain outputs for controlling a DC-DC converter.

6. A power good flag pin allows an accurate power good delay to be programmed and provides the option of

driving a power good indicator LED.

7. Input line over voltage protection for downstream circuits, including the DC-DC converter.

DC-DC Converter Selection

A PD designed with LM5073 can be optimized for a variety of applications by selecting the DC-DC converter

from a wide range of topologies. Topology selection enables several design trade-offs including efficiency,

complexity, and cost.

For example, the LM5025 controller for the Active Clamp Forward topology can be paired with the LM5073 for

increased efficiency, especially at higher power levels. In cases where isolation is not required an LM5576

regulator with a built in buck switch provides a simple, low cost solution.

The 100V capability of the LM5073 protects against input voltage transients, especially in the case of a hot

swapping front auxiliary power. The LM5073 has built-in over-voltage protection such that a DC-DC converter

with input voltage rating as low as 65V can be safely used.

The DC-DC converter must have a soft start feature to control the input current during startup. The soft-start

process reduces the surge of inrush current and eliminates any tendency of the output voltage to overshoot

during startup. The converter should be started slowly enough such that the input current does not exceed the

PD interface hot swap MOSFET DC current limit or the current limit of the PSE, otherwise the PD will not start

correctly.

Modes of Operation

Per the IEEE 802.3af specification, when a PD is connected to a PoE system it transitions through several

operating modes in sequence including detection, classification (optional), turn-on inrush, and normal DC

operation. Each operating mode corresponds to a specific voltage range supplied from the PSE. Figure 8 shows

the IEEE 802.3af specified sequence of operating modes and the corresponding PD input voltages at the RJ-45

connector.

Current steering diode bridges are required for the PD interface to accept all allowable connections and polarities

of PoE voltage from the RJ-45 connector (see the example application circuit in Figure 17). The bridge voltage

drop will reduce the input voltage sensed by the LM5073. To ensure full compliance to the specification in all

operating modes, the LM5073 takes into account the voltage drop across the bridge diodes and responds

appropriately to the voltage received from the PoE cable. Table 1 presents the response in each operating mode

to voltages at the PD input connector and between the VIN and VEE pins.

Copyright © 2007–2013, Texas Instruments Incorporated

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