Datasheet

LTC4269-1
20
42691fc
operating current, the pull-down capability of the T2P pin,
and the choice of V
+
. V
+
for example can come from the
PoE supply rail (which the LTC4269-1 V
PORTP
is tied to),
or from the voltage source that supplies power to the DC/
DC converter. Option 1 has the advantage of not drawing
power unless T2P is declared active.
Shutdown Interface
To corrupt the signature resistance, the SHDN pin can be
driven high with respect to V
PORTN
. If unused, connect
SHDN directly to V
PORTN
.
Auxiliary Power Source
In some applications, it is desirable to power the PD from
an auxiliary power source such as a wall adapter.
Auxiliary power can be injected into an LTC4269-1-based
PD at the input of the LTC4269-1 V
PORTN
, at V
NEG
, or even
the power supply output. In addition, some PD application
may desire auxiliary supply dominance or may be con-
fi gured 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 LTC4269-1 does not disconnect power
to the output until the auxiliary supply exceeds 36V.
This confi guration is an auxiliary-dominant confi guration.
That is, the auxiliary power source supplies the power even
if PoE power is already present. This confi guration 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 13.0W as a Type 1 PD until it recognizes a Type 2
PSE. Initializing PD operation in 13.0W 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
13.0W 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 0.05F. If one of these conditions
is not met, the PSE may disconnect power to the PD.
APPLICATIONS INFORMATION
Figure 9. Auxiliary Power Dominant PD Interface Example
T1
42691 F09
TVS
TO PHY
36V
100k
10k
10k
D1
BR1
+
–
BR2
+
–
0.1µF
100V
C1
V
PORTP
LTC4269-1
V
PORTN
SHDN
V
NEG
GND
+
–
ISOLATED
WALL
TRANSFORMER
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
1
7
8
5
4
SPARE
–
SPARE
+