LTC4267CGNPBF LINEAR TECH, LTC4267CGNPBF Datasheet - Page 17

• Manufacturer Part Number: LTC4267CGNPBF
• Manufacturer: LINEAR TECH

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 has several different modes of operation
based on the voltage present between V
pins. The forward voltage drop of the input diodes in a PD
design subtracts from the input voltage and will affect the
transition point between modes. When using the LTC4267,
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
designer should determine which power category the PD
APPLICATIO S I FOR ATIO
PORTN
(Figure 4) sets the value of the load current. The
U
U
W
CLASS
PORTN
)
and V
U
CLASS
PORTP
to
falls into and then select the appropriate value of R
from Table 2. If a unique load current is required, the value
of R
where I
classifi cation and is given in the electrical specifi cations.
The R
the overall accuracy of the classifi cation circuit. Resistor
power dissipation will be 50mW maximum and is transient
so heating is typically not a concern. In order to maintain
loop stability, the layout should minimize capacitance at
the R
by fl oating the R
shorted to V
sifi cation circuit to attempt to source very large currents
and quickly go into thermal shutdown.
Power Good Interface
The ⎯ P ⎯ W ⎯ R ⎯ G ⎯ D signal is controlled by a high voltage, open-
drain transistor. The designer has the option of using this
signal to enable the onboard switching regulator through
the I
interface circuits for controlling the switching regulator
are shown in Figure 7.
In some applications, it is desirable to ignore intermittent
power bad conditions. This can be accomplished by in-
cluding capacitor C15 in Figure 7 to form a lowpass fi lter.
With the components shown, power bad conditions less
than about 200µs will be ignored. Conversely, in other
applications it may be desirable to delay assertion of
⎯ P ⎯ W ⎯ R ⎯ G ⎯ D to the switching regulator using C
as shown in Figure 7.
It is recommended that the designer use the power
good signal to enable the switching regulator. Using
⎯ P ⎯ W ⎯ R ⎯ G ⎯ D ensures the capacitor C1 has reached within
1.5V of the fi nal value and is ready to accept a load. The
LTC4267 is designed with wide power good hysteresis
to handle sudden fl uctuations in the load voltage and
current without prematurely shutting off the switching
regulator. Please refer to the Power-Up Sequencing of the
Application Information section.
R
CLASS
CLASS
TH
CLASS
CLASS
/RUN or the P
IN_CLASS
can be calculated as:
= 1.237V/(I
node. The classifi cation circuit can be disabled
resistor must be 1% or better to avoid degrading
PORTN
is the LTC4267 IC supply current during
CLASS
as this would force the LTC4267 clas-
DESIRED
VCC
pin. The R
pins. Examples of active-high
– I
IN_CLASS
CLASS
pin should not be
LTC4267
)
PVCC
17
or C17
CLASS
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