Datasheet

TPS23757
www.ti.com SLVS948D –JULY 2009–REVISED NOVEMBER 2013
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 TPS23757 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 a flyback
with a driven output synchronous rectifier. The TPS23757 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 TPS23757 are shown below. 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
• TPS23757EVM: Evaluation Module for TPS23757, SLVU318
Input Bridges and Schottky Diodes
Using Schottky diodes instead of PN junction diodes for the PoE input bridges and D
VDD
will reduce the loss of
this function by about 30%. There are however some things to consider when using them.
The IEEE standard specifies a maximum backfeed voltage of 2.8 V . A 100 kΩ resistor is placed between the
unpowered pairs and the voltage is measured across the resistor. Schottky diodes often have a higher reverse
leakage current than PN diodes, making this a harder requirement to meet. Use conservative design for diode
operating temperature, select lower-leakage devices where possible, and match leakage and temperatures by
using packaged bridges to help with this.
Schottky diode leakage current and lower dynamic resistance can impact the detection signature. Setting
reasonable expectations for the temperature range over which the detection signature is accurate is the simplest
solution. Increasing R
DEN
slightly may also help meet the requirement.
Schottky diodes have proven less robust to the stresses of ESD transients, failing as a short or becoming leaky.
Care must be taken to provide adequate protection in line with the exposure levels. This protection may be as
simple as ferrite beads and capacitors.
A general recommendation for the input rectifiers are 1 A or 2 A, 100 V rated discrete or bridge diodes.
Protection, D1
A TVS, D
1
, across the rectified PoE voltage per Figure 24 must be used. An SMAJ58A, or a part with equal to or
better performance, is recommended for general indoor applications. If an adapter is connected from V
DD1
to
RTN, as in ORing option 2 above, voltage transients caused by the input cable inductance ringing with the
internal PD capacitance can occur. Adequate capacitive filtering or a TVS must limit this voltage to be within the
absolute maximum ratings. Outdoor transient levels or special applications require additional protection.
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