LM5072 datasheet(10/24 Pages) NSC

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Specialized Block Diagrams (Continued)

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:

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.

The integration of the PD signature resistor and other

functions including programmable inrush current limit,

input voltage under-voltage lock-out (UVLO), PD classi-

fication, and thermal shutdown simplifies PD implemen-

tation.

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.

DC current limit is programmable and adjustable to sup-

port 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 fre-

quency between 50 kHz to 500 kHz, a bias supply (V

CC)

under-voltage lock-out circuit, and a programmable soft-start

circuit. These features greatly simplify the design and imple-

mentation of single ended topologies like the flyback, for-

ward 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 compensa-

tion is provided in the LM5072-80 version.

20184616

FIGURE 2. PWM Controller Block Diagram

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