Datasheet

TPS23751/2
D1
C1
R
DEN
R
CLS
From Ethernet
Transformers
V
DD
V
SS
CLS
DEN
RTN
Power
Circuit
Adapter
Option 3
Adapter
Option 2
Adapter
Option 1
+
V
POE
±
Low Voltage
Output
From Spare Pairs
or Transformers
TPS23751
TPS23752
www.ti.com SLVSB97C –JULY 2012–REVISED JANUARY 2014
Figure 28. Oring Configurations
The IEEE standards require that the Ethernet cable be isolated from ground and all other system potentials. The
adapter must meet a minimum 1500 Vac dielectric withstand test between the output and all other connections
for ORing options 1 and 2. The adapter only needs this isolation for option 3 if it is not provided by the converter.
Adapter ORing diodes are shown for all the options to protect against a reverse voltage adapter, a short on the
adapter input pins, or damage to a low-voltage adapter. ORing is sometimes accomplished with a MOSFET in
option 3.
Using DEN to Disable PoE
The DEN pin may be used to turn the PoE hotswap switch off by pulling it to V
SS
while in the operational state, or
to prevent detection when in the idle state. A low voltage on DEN forces the hotswap MOSFET off during normal
operation.
ORing Challenges
Preference of one power source presents a number of challenges. Combinations of adapter output voltage
(nominal and tolerance), power insertion point, and which source is preferred determine solution complexity.
Several factors adding to the complexity are the natural high-voltage selection of diode ORing (the simplest
method of combining sources), the current limit implicit in the PSE, and PD inrush and protection circuits
(necessary for operation and reliability). Creating simple and seamless solutions is difficult, if not impossible, for
many of the combinations. However, the TPS23751 and TPS23752 offer several built-in features that simplify
some combinations.
Several examples demonstrate the limitations inherent in ORing solutions. Diode ORing a 48 V adapter with PoE
(option 1) presents the problem that either source may have the higher voltage. A blocking switch would be
required to assure that one source dominates. A second example combines a 12 V adapter with PoE using
option 2. The converter draws approximately four times the current at 12 V from the adapter than it does from
PoE at 48 V. Transition from adapter power to PoE may demand more current than can be supplied by the PSE.
The converter must be turned off while the C
IN
capacitance charges, with a subsequent converter restart at the
higher voltage and lower input current. A third example involves the loss of the MPS when running from the
adapter, causing the PSE to remove power from the PD. If ac power is then lost, the PD stops operating until the
PSE detects and powers the PD.
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