Datasheet

TPS23754
58V
0.1uF
R
DEN
R
CLS
From Ethernet
Transformers
V
DD
V
SS
CLS
DEN
LowVoltage
Output
RTN
From Spare
Pairs or
Transformers
Power
Circuit
Adapter
Option3
Adapter
Option2
Adapter
Option1
V
DD1
OptionalforPoEPriority
TPS23757
SLVS948D –JULY 2009–REVISED NOVEMBER 2013 www.ti.com
Adapter ORing
Many PoE-capable devices are designed to operate from either a wall adapter or PoE power. A local power
solution adds cost and complexity, but allows a product to be used if PoE is not available in a particular
installation. While most applications only require that the PD operate when both sources are present, the
TPS23757 supports forced operation from either of the power sources. Figure 23 illustrates three options for
diode ORing external power into a PD. Only one option would be used in any particular design. Option 1 applies
power to the TPS23757 PoE input, option 2 applies power between the TPS23757 PoE section and the dc/dc
converter, and option 3 applies power to the output side of the converter. Each of these options has advantages
and disadvantages. Many of the basic ORing configurations and discussion contained in application note
Advanced Adapter ORing Solutions using the TPS23753 (literature number SLVA306), apply to the TPS23757.
Figure 23. 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, and damage to a low-voltage adapter. ORing is sometimes accomplished with a MOSFET in
option 3.
PPD ORing Features
The TPS23757 provides several additional features for the ORing based on the multifunction PPD pin. These
include APb signaling of an option 1 adapter, use of a 24 V adapter (reduced output power) for option 1, and use
of PoE as a power backup in conjunction with option 2. See the Advanced ORing Techniques section.
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 on DEN forces the hotswap MOSFET OFF during normal
operation. Additional information is available in Advanced Adapter ORing Solutions using the TPS23753
(literature number SLVA306).
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 TPS23757 offers several built-in features that simplify some
combinations.
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