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
TPS23754
TPS23754-1
TPS23756
www.ti.com SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013
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
TPS23754 device supports forced operation from either of the power sources. Figure 27 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 TPS23754 device PoE input, option 2 applies power between the TPS23754 device PoE
section and the power circuit, 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 TPS23754 device.
Figure 27. 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 TPS23754 device provides several additional features to ease ORing based on the multifunction PPD pin
(not available on TPS23754-1 device). These include T2P 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.
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 the Advanced Adapter ORing Solutions using the TPS23753
(literature number SLVA306) application report.
Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 25