Datasheet

RTN
CDB
SS
GND
UCC3809
TPS2379
C
SS
TPS2379
www.ti.com SLVSB98 –MARCH 2012
APPLICATION INFORMATION
INPUT BRIDGES AND SCHOTTKY DIODES
Using Schottky diodes instead of PN junction diodes for the PoE input bridges will reduce the power dissipation
in these devices 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. To compensate, use conservative
design for diode operating temperature, select lower-leakage devices where possible, and match leakage and
temperatures by using packaged bridges.
Schottky diode leakage currents and lower dynamic resistances can impact the detection signature. Setting
reasonable expectations for the temperature range over which the detection signature is accurate is the simplest
solution. Increasing RDEN slightly may also help meet the requirement.
Schottky diodes have proven less robust to the stresses of ESD transients than PN junction diodes. After
exposure to ESD, Schottky diodes may become shorted or leak. 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.
As a general recommendation, use 1 A or 2 A, 100 V rated discrete or bridge diodes for the input rectifiers.
Protection, D1
A TVS, D1, across the rectified PoE voltage per Figure 1 must be used. A SMAJ58A, or equivalent, is
recommended for general indoor applications. Adequate capacitive filtering or a TVS must limit input transient
voltage to within the absolute maximum ratings. Outdoor transient levels or special applications require additional
protection.
Capacitor, C
1
The IEEE 802.3at standard specifies an input bypass capacitor (from VDD to VSS) 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 23.75 kΩ and 26.25 kΩ, or
25 kΩ ± 5%. A resistor of 24.9 kΩ ± 1% is recommended for R
DEN
.
Classification Resistor, RCLS
Connect a resistor from CLS to VSS to program the classification current according to the IEEE 802.3at
standard. The class power assigned should correspond to the maximum average power drawn by the PD during
operation. Select R
CLS
according to Table 1. For a high power design, choose class 4 and R
CLS
= 63.4 Ω.
CDB Pin Interface
The CDB pin can be used to inhibit downstream converter start up by keeping the soft start pin low. Figure 25
shows an example where CDB connects to the SS pin of a UCC3809 DC/DC controller. Since CDB is an open-
drain output, it will not affect the soft start capacitor charge time when it de-asserts. Another common use of the
CDB pin is to enable a converter with an active-high enable input. In this case, CDB may require a pullup resistor
to either VDD, or to a bias supply, depending on the requirements of the controller enable pin.
Figure 25. CDB Interface
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