VIN

RAUX

VEE

OUT

RT

CS

FB

COMP

SS

RTN

ARTN

DCCL

RCLASS

nPGOOD

VCC

+PoE

+ FAUX

- FAUX

- PoE

ICL_FAUX

+VOUT

LM5072

0.1 PF

CPGOOD(nF) =

0.05mA + ILED (mA)

1mA

0.25 ms

x

tPG_Delay (ms)

x 100 nF

LM5072

www.ti.com

SNVS437D – MARCH 2006 – REVISED APRIL 2013

(3)

For example, selecting 1000 nF for CPGOOD, the delay time will be 50 ms if no LED is used and about 0.83 ms

when an LED, drawing 3 mA, is used. The delay required for continued operation will depend on the amplitude of

the transient, the DC current limit, the load, and the total amount of input capacitance. Note that this delay does

not ensure continued operation. If the hot swap MOSFET is in current limit for an extended period, it may cause

a thermal limit condition. This will result in a complete shutdown of the switching regulator, though no elements in

the system will be permanently damaged and normal operation will resume momentarily.

The Power Good status will also affect the default DC current limit. Should the sensed drain to source voltage of

the hot swap MOSFET (from ARTN to VEE) exceed 2.5V, the LM5072 will increase the DC current limit from the

default 440 mA to 540 mA, thus allowing the PD to continue operation through the transient event. This higher

current limit will remain in effect until one of the following events occur:

1. the duration of loss of Power Good status exceeds tPG_Delay, at which time the PWM controller will be

disabled,

2. the increased power dissipation in the hot swap MOSFET causes a thermal limit condition as previously

discussed, or

3. the MOSFET drain to source voltage falls below 1.5V to re-establish Power Good status. Under this

condition, the LM5072 will revert back to the default 440 mA DC current limit once Power Good status is

restored. Note that if the DC current limit has been programmed externally with RDCCL (see the DC Current

Limit Programming section), the DC current limit will remain at the programmed level even when the Power

Good status is lost.

Auxiliary Power Options

The LM5072 based PD can receive power from auxiliary sources like AC adapters and solar cells in addition to

the PoE enabled network. This is a desirable feature when the total system power requirements exceed the

PSE’s load capacity. Furthermore, with the auxiliary power option the PD can be used in a standard Ethernet

(non-PoE) system.

For maximum versatility, the LM5072 accepts two different auxiliary power configurations. The first one, shown in

Figure 23, is the front auxiliary (FAUX) configuration in which the auxiliary source is “diode OR’d” with the PoE

potential received from the Ethernet connector. The second configuration, shown in Figure 24, is the rear

auxiliary (RAUX) option in which the auxiliary power bypasses the PoE interface and is connected directly to the

input of the DC-DC converter through a diode. The FAUX option is desirable if the auxiliary power voltage is

similar to the PoE input voltage. However, when the auxiliary supply voltage is much lower than the PoE input

voltage, the RAUX option is more favorable because the current from the auxiliary supply is not limited by the hot

swap MOSFET DC current limit. A comparison of the FAUX and RAUX options is presented in Table 3. Note the

ICL_FAUX and RAUX pins are not reverse voltage protected. If complete reverse protection is desired, series

blocking diodes are necessary.

Figure 23. The FAUX Configuration

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