LM5072 datasheet(14/24 Pages) NSC

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Inrush Current Limit Programming

(Continued)

will turn on the hot swap MOSFET in a constant current

mode. The default, pre-programmed inrush current of 150

mA can be selected by simply leaving the ICL_FAUX pin

open.

To adjust the capacitor charging time for a particular appli-

cation requirement, the inrush limit can be programmed to

any value between 150 and 400 mA with an external resistor

ICL) between the ICL_FAUX and VEE pins, as shown in

Figure 7. The relationship between the R

ICL value and the

desired inrush current limit I

INRUSH satisfies the following

equation:

The inrush current causes a voltage drop along the PoE

Ethernet cable (20

Ω maximum) that reduces the input volt-

age sensed by the LM5072. To avoid erratic turn-on (hic-

cups), I

INRUSH should be programmed such that the input

voltage drop due to cable resistance does not exceed the

IN-UVLO hysteresis (6V minimum).

DC Current Limit Programming

The LM5072 provides a default DC current limit of 440 mA

nominal. This default limit can be selected by leaving the

DCCL pin open.

The LM5072 allows the DC current limit to be programmed

within the range from 150 mA to 800 mA. Figure 8 shows the

method to program the DC current limit with an external

resistor, R

DCCL. The relationship between the RDCCL value

and the desired DC current limit I

DC satisfies the following

equation:

The maximum recommended DC current limit is 800 mA.

While thermal analysis should be a standard part of the

module development process, it may warrant additional at-

tention if the DC current limit is programmed to values in

excess of 400 mA. This analysis should include evaluations

of the dissipation capability of LM5072 package, heat sinking

properties of the PC Board, ambient temperature, and other

heat dissipation factors of the operating environment.

Power Good and Regulator Startup

The Power Good status indicates that the circuit is ready for

PWM controller startup to occur. It is established when the

input capacitors are fully charged through the hot swap

MOSFET. Since the hot swap MOSFET is in series with the

input capacitors of the DC-DC converter, its drain-to-source

voltage decreases as the charging occurs. Power Good is

indicated when the following two conditions are met: the

MOSFET drain-to-source voltage drops below 1.5V (with 1V

hysteresis), and the gate-to-source voltage is greater than

5V. Circuitry internal to the LM5072 monitors both the drain

and gate voltages (see Figure 1), and issues the Power

Good status flag by pulling down the nPGOOD pin to a logic

low level relative to the ARTN pin.

The nPGOOD circuitry consists of a 2.5V comparator, a

130

Ω pull down MOSFET, and a 50 µA pull up current

source, as shown in Figure 9. Once the Power Good status

is established, the nPGOOD pin voltage will be pulled down

quickly by the MOSFET, and the PWM controller will start as

soon as the nPGOOD pin voltage drops below the 2.5V

threshold.

20184622

FIGURE 7. Input Inrush Limit Programming via R

ICL

20184624

FIGURE 8. Input DC Current Limit Programming via

DCCL

LM5072 Datasheet(PDF) 14 Page - National Semiconductor (TI)

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