Datasheet

VIN
RAUX
VEE
OUT
RT
CS
FB
COMP
SS
RTN
ARTN
DCCL
RCLASS
nPGOOD
VCC
+PoE
+ FAUX
- FAUX
-PoE
ICL_FAUX
+V
OUT
LM5072
0.1 PF
C
PGOOD
(nF) =
0.05mA + I
LED (mA)
1mA
0.25 ms
x
t
PG_Delay (ms)
x 100 nF
LM5072
www.ti.com SNVS437D –MARCH 2006–REVISED APRIL 2013
(3)
For example, selecting 1000 nF for C
PGOOD
, 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 V
EE
) 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 t
PG_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 R
DCCL
(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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