Datasheet

VIN
RAUX
VEE
OUT
RT
CS
FB
COMP
SS
RTN
ARTN
ICL_ FAUX
DCCL
RCLASS
nPGOOD
VCC
+PoE
-RAUX
+RAUX
0.1 PF
LM5072
-PoE
+V
OUT
LM5072
SNVS437D –MARCH 2006–REVISED APRIL 2013 www.ti.com
Figure 24. The RAUX Configuration
Table 3. Comparison Between FAUX and RAUX Operation
Tradeoff FAUX Operation RAUX Operation
Hot Swap Protection / Current Limit Automatically provided by the hot swap Requires a series resistor to limit the inrush current
Protection MOSFET. during hot swap.
Minimum Auxiliary Voltage Limited to 18V by the signature detection Only limited by 9V minimum input requirement.
(at the IC pins) mode, or by the power requirement
(current limit).
Auxiliary Dominance Over PoE Cannot be forced without external Can be forced with appropriate RAUX pin
components. configuration.
Use of nPGOOD Pin as “Powered from Not applicable as power is delivered Supported.
PoE” Indicator through the hot swap interface in both
PoE and FAUX modes.
Transient Protection Excellent due to active MOSFET current Fair due to passive resistor current limit.
limit.
The term “Auxiliary Dominance” mentioned in Table 3 means that when the auxiliary power source is connected,
it will always power the PD regardless of the state of PoE power. “Aux dominance” is achievable only with the
RAUX option, as noted in the table.
If the PD is not designed for aux dominance, either the FAUX or RAUX power sources will deliver power to the
PD only under the following two conditions:
1. If auxiliary power is applied before PoE power, it will prevent the PD’s detection by the PSE and will supply
power indefinitely. This occurs because the PoE input bridge rectifiers will be reverse biased, so no detection
signature will be observed. Under this condition, when the auxiliary supply is removed, power will not be
maintained because it will take some time for the PSE to perform signature detection and classification
before it will supply power.
2. If auxiliary power is applied after PoE power is already present but has a higher voltage than PoE, it may
assume power delivery responsibility. Under the second case, if the supplied voltages are comparable, the
load current may be shared inversely proportional to the respective output impedances of each supply. (The
output impedance of the PSE supply is increased by the cable series resistance).
If PoE power is applied first and has a higher voltage than the non-dominant aux power source, it will continue
powering the PD even when the aux power source becomes available. In this case, should PoE power be
removed, the auxiliary source will assume power delivery and supply the DC-DC loads without interruption.
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