LM5070 datasheet(10/17 Pages) NSC

10 / 17 page

Detailed Operating Description

The LM5070 power interface port and pulse width modula-

tion (PWM) controller provides a complete integrated solu-

tion for Powered Devices (PD) that connect into Power over

Ethernet (PoE) systems. Major features of the PD interface

portion of the IC include detection, classification, program-

mable inrush current limit, thermal limit, programmable und-

ervoltage lockout, and current limit monitoring. The device

also includes a high-voltage start-up bias regulator that op-

erates over a wide input range up to 75V. The switch mode

power supply (SMPS) control portion of the IC includes

power good sensing, V

CC regulator under-voltage lockout,

cycle-by-cycle current limit, error amplifier, slope compensa-

tion, softstart, and oscillator sync capability. This high speed

BiCMOS IC has total propagation delays less than 100ns

and a 1MHz capable oscillator programmed by a single

external resistor. The LM5070 PWM controller provides

current-mode control for dc-dc converter topologies requiring

a single drive output, such as Flyback and Forward topolo-

gies. The LM5070 PWM enables all of the advantages of

current-mode control including line feed-forward, cycle-by-

cycle current limit and simplified loop compensation. The

oscillator ramp is internally buffered and added to the PWM

comparator input ramp to provide slope compensation nec-

essary for current mode control at duty cycles greater than

50% (-80 suffix only).

Modes of Operation

The LM5070 PD interface is designed to provide a fully

compliant IEEE 802.3af system. As such, the modes of

operation take into account the barrel rectifiers often utilized

to correctly polarize the dc input from the Ethernet cable.

Table 1 shows the LM5070 operating modes and associated

input voltage range.

20120005

FIGURE 3. PWM Controller Block Diagram

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