ltc4267cgn-1 Linear Technology Corporation, ltc4267cgn-1 Datasheet - Page 17

• Manufacturer Part Number: ltc4267cgn-1
• Description: Power Over Ethernet Ieee 802.3af Pd Interface With Integrated Switching Regulator
• Manufacturer: Linear Technology Corporation

Diode Bridge
IEEE 802.3af allows power wiring in either of two confi gu-
rations: on the TX/RX wires or via the spare wire pairs in
the RJ45 connector. The PD is required to accept power in
either polarity on either the main or spare inputs; therefore
it is common to install diode bridges on both inputs in
order to accommodate the different wiring confi gurations.
Figure 9 demonstrates an implementation of these diode
bridges. The IEEE 802.3af specifi cation also mandates
that the leakage back through the unused bridge be less
than 28μA when the PD is powered with 57V.
The IEEE standard includes an AC impedance requirement
in order to implement the AC disconnect function. Capaci-
tor C14 in Figure 9 is used to meet this AC impedance
requirement. A 0.1μF capacitor is recommended for this
application.
The LTC4267-1 has several different modes of opera-
tion based on the voltage present between V
V
in a PD design subtracts from the input voltage and will
APPLICATIO S I FOR ATIO
PORTP
pins. The forward voltage drop of the input diodes
U
Figure 9. PD Front End with Isolation Transformer, Diode Bridges and Capacitor
U
W
PORTN
U
and
affect the transition point between modes. When using
the LTC4267-1, it is necessary to pay close attention to
this forward voltage drop. Selection of oversized diodes
will help keep the PD thresholds from exceeding IEEE
specifi cations.
The input diode bridge of a PD can consume over 4%
of the available power in some applications. It may be
desirable to use Schottky diodes in order to reduce power
loss. However, if the standard diode bridge is replaced
with a Schottky bridge, the transition points between the
modes will be affected. Figure 10 shows a technique for
using Schottky diodes while maintaining proper threshold
points to meet IEEE 802.3af compliance. D13 is added to
compensate for the change in UVLO turn-on voltage caused
by the Schottky diodes and consumes little power.
Classifi cation Resistor Selection (R
The IEEE specifi cation allows classifying PDs into four
distinct classes with class 4 being reserved for future use
(Table 2). An external resistor connected from R
V
PORTN
(Figure 4) sets the value of the load current. The
LTC4267-1
CLASS
)
CLASS
17
42671f
to