Datasheet
M1
R
CS
C
OUT2
D
VC1
GATE
RTN, COM
ARTN
V
C
CS
C
VC
CTL
V
B
58V
0.1uF
R
DEN
From Ethernet
Pairs 1,2
V
SS
C
IN
R
CTL
C
CTL
From Ethernet
Pairs 3,4
CLS
DEN
FRS
T2P
R
CLS
R
T2P_OUT
R
FRS
V
B
C
VB
R
FBU
R
FBL
TLV431
R
OB
C
IZ
V
OUT
PAD
V
DD
GAT2
M2
R
T2P
D
A
R
APD2
R
APD1
PPD
Adapter
DT
R
BLNK
APD
R
DT
BLNK
V
DD1
C
OUT1
L
OUT
T1
T2
C
IO
V
T2P_OUT
TPS23754
TPS23754-1
TPS23756
SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013
www.ti.com
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 TPS23754 device offers several built-in features that simplify some
combinations.
Several examples will demonstrate the limitations inherent in ORing solutions. Diode ORing, a 48-V adapter with
PoE (option 1), presents the problem that either source might be higher. A blocking switch would be required to
assure which source was active. A second example is combining a 12-V adapter with PoE using option 2. The
converter will draw 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 C
IN
capacitance charges, with a subsequent converter restart at the higher
voltage and lower input current. A third example is use of a 12-V adapter with ORing option 1. The PD hotswap
would have to handle four times the current, and have 1/16 the resistance (be 16 times larger) to dissipate equal
power. A fourth example is that MPS is lost when running from the adapter, causing the PSE to remove power
from the PD. If AC power is then lost, the PD will stop operating until the PSE detects and powers the PD.
APPLICATION INFORMATION
The TPS23754 device will support many power supply topologies that require a single PWM gate drive or two
complementary gate drives and will operate with current-mode control. Figure 1 provides an example of an active
clamp forward converter that uses the second gate driver to control M2, the active element in the clamp. GAT2
may also be used to drive a synchronous rectifier as demonstrated in Figure 28. The TPS23754 may be used in
topologies that do not require GAT2, which may be disabled to reduce its idling loss.
Selecting a converter topology along with a design procedure is beyond the scope of this applications section.
Examples to help in programming the TPS23754 are shown in the following list. Additional special topics are
included to explain the ORing capabilities, frequency dithering, and other design considerations.
For more specific converter design examples refer to the following application notes:
• Designing with the TPS23753 Powered Device and Power Supply Controller, SLVA305
• Understanding and Designing an Active Clamp Current Mode Controlled Converter Using the UCC2897A.
SLUA535
• Advanced Adapter ORing Solutions using the TPS23753, SLVA306A
• TPS23754EVM-420 EVM: Evaluation Module for TPS23754, SLVU301
• TPS23754EVM-383 EVM: Evaluation Module for TPS23754, SLVU304
Figure 28. Driven Synchronous Flyback
26 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated