Datasheet

C
1
0.1mF
R
DEN
R
CLS
From Ethernet
Transformers
V
DD
V
SS
CLS
From Spare
Pairs or
Transformers
DEN
PPD
D
VDD
V
DD1
RTN
COM
ARTN
C
IN
D
1
58V
D
RTN
58V
C
VDD
0.01mF
TPS23754
TPS23754-1
TPS23756
www.ti.com SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013
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%. However, there are 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 29 must be used. TI recommends a SMAJ58A, or a part
with equal to or better performance, 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.
Use of diode D
VDD
for PoE priority may dictate the use of additional protection around the TPS23754. ESD
events between the PD power inputs, or the inputs and converter output, cause large stresses in the hotswap
MOSFET if D
VDD
becomes reverse biased and transient current around the TPS23754 is blocked. The use of
C
VDD
and D
RTN
in Figure 29 provides additional protection should over-stress of the TPS23754 device be an
issue. A SMAJ58A would be a good initial selection for D
RTN
. Individual designs may have to tune the value of
C
VDD
.
Figure 29. Example of Added ESD Protection for PoE Priority
Capacitor, C
1
The IEEE 802.3at standard specifies an input bypass capacitor (from V
DD
to V
SS
) of 0.05 μF to 0.12 μF. Typically
a 0.1-μF, 100-V, 10% ceramic capacitor is used.
Detection Resistor, R
DEN
The IEEE 802.3at standard specifies a detection signature resistance, R
DEN
between 23.7 kΩ and 26.3 kΩ, or 25
kΩ ± 5%. Choose an R
DEN
of 24.9 kΩ.
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