Datasheet

LM5072

www.ti.com

SNVS437D –MARCH 2006–REVISED APRIL 2013

DESCRIPTION OF OPERATION AND APPLICATIONS INFORMATION

The LM5072 integrates a fully IEEE 802.3af compliant PD interface and PWM controller in a single integrated

circuit, providing a complete and low cost power solution for devices that connect to PoE systems. The

implementation requires a minimal number of external components.

The LM5072’s Hot Swap PD interface provides four major advantages:

1. An input voltage rating up to 100V that 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 and other functions including programmable inrush current limit,

input voltage under-voltage lock-out (UVLO), PD classification, and thermal shutdown simplifies PD

implementation.

3. The PD interface and PWM controller accept power from auxiliary sources including AC adapters and solar

cells in various configurations and over a wide range of input voltages. Auxiliary power input can be

programmed to be dominant over PoE power.

4. DC current limit is programmable and adjustable to support PoE applications requiring input currents up to

700 mA.

The LM5072 includes an easy to use PWM controller based on the peak current mode control technique. Current

mode control provides inherent advantages such as line voltage feed-forward, cycle-by-cycle current limit, and

simplified closed-loop compensation. The controller’s PWM gate driver is capable of sourcing and sinking peak

currents of 800 mA to directly drive the power MOSFET switch of the DC-DC converter. The PWM controller also

contains a high gain, high bandwidth error amplifier, a high voltage startup bias regulator, a programmable

oscillator for a switching frequency between 50 kHz to 500 kHz, a bias supply (V

) under-voltage lock-out

circuit, and a programmable soft-start circuit. These features greatly simplify the design and implementation of

single ended topologies like the flyback, forward and buck.

The LM5072 is available in two versions, the LM5072-50 and LM5072-80. As indicated in the suffix of the part

number, the maximum duty cycle of each device is limited to 50% and 80%, respectively. Internal PWM controller

slope compensation is provided in the LM5072-80 version.

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, normal operation, and power

removal. Each operating mode corresponds to a specific PoE voltage range fed through the Ethernet cable.

Figure 16 shows the IEEE 802.3af specified sequence of operating modes and the corresponding PD input

voltages.

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 circuits in Figure 31, Figure 32, Figure 33

and Figure 34). The bridge will cause some reduction of the input voltage sensed by the LM5072. To ensure full

compliance to the specification in all operating modes, the LM5072 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 across the VIN and VEE pins.

Product Folder Links: LM5072