Datasheet

LTC4278
21
4278fc
for PoE dominance. Furthermore, PD applications may
also opt for a seamless transition — that is, without power
disruption — between PoE and auxiliary power.
The most common auxiliary power option injects power at
V
NEG
. Figure 9 presents an example of this application. In
this example, the auxiliary port injects 48V onto the line via
diode D1. The components surrounding the SHDN pin are
selected so that the LTC4278 does not disconnect power
to the output until the auxiliary supply exceeds 36V.
This configuration is an auxiliary-dominant configuration.
That is, the auxiliary power source supplies the power even
if PoE power is already present. This configuration also
provides a seamless transition from PoE to auxiliary power
when auxiliary power is applied, however, the removal of
auxiliary power to PoE power is not seamless.
Contact Linear Technology applications support for detail
information on implementing a custom auxiliary power
supply.
IEEE 802.3at SYSTEM POWER-UP REQUIREMENT
Under the IEEE 802.3at standard, a PD must operate under
12.95W in accordance with IEEE 802.3at standard until it
recognizes a Type 2 PSE. Initializing PD operation in 12.95W
mode eliminates interoperability issue in case a Type 2
PD connects to a Type 1 PSE. Once the PD recognizes a
Type 2 PSE, the IEEE 802.3at standard requires the PD to
wait 80ms in 12.95W operation before 25.5W operation
can commence.
MAINTAIN POWER SIGNATURE
In an IEEE 802.3af/at system, the PSE uses the maintain
power signature (MPS) to determine if a PD continues to
require power. The MPS requires the PD to periodically
draw at least 10mA and also have an AC impedance less
than 26.25k in parallel with 0.05μF. If one of these condi-
tions is not met, the PSE may disconnect power to the PD.
SWITCHING REGULATOR OVERVIEW
The LTC4278 includes a current mode converter designed
specifically for use in an isolated flyback topology employing
synchronous rectification. The LTC4278 operation is similar
to traditional current mode switchers. The major difference
is that output voltage feedback is derived via sensing the
output voltage through the transformer. This precludes
the need of an opto-isolator in isolated designs, thus
greatly improving dynamic response and reliability. The
LTC4278 has a unique feedback amplifier that samples a
transformer winding voltage during the flyback period and
uses that voltage to control output voltage. The internal
blocks are similar to many current mode controllers.
The differences lie in the feedback amplifier and load
APPLICATIONS INFORMATION
Figure 9. Auxiliary Power Dominant PD Interface Example
T1
4278 F09
TVS
TO PHY
36V
100k
10k
10k
D1
BR1
+
–
BR2
+
–
0.1µF
100V
C1
V
PORTP
LTC4278
V
PORTN
SHDN
V
NEG
GND
+
–
ISOLATED
WALL
TRANSFORMER
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
1
7
8
5
4
SPARE
–
SPARE
+