LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 21

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
Shutdown Interface
To corrupt the signature resistance, the SHDN pin can be
driven high with respect to V
SHDN directly to V
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
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
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
NEG
. Figure 9 presents an example of this application. In
RJ45
1
2
3
6
4
5
7
8
PORTN
TX
TX
RX
RX
+
–
+
–
SPARE
SPARE
.
+
–
TRANSFORMER
PORTN
ISOLATED
T1
WALL
Figure 9. Auxiliary Power Dominant PD Interface Example
PORTN
. If unused, connect
+
–
, at V
TO PHY
NEG
, or even
BR1
BR2
+
–
+
–
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 rec-
ognizes 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 conditions
is not met, the PSE may disconnect power to the PD.
TVS
100k
10k
10k
36V
0.1μF
100V
SHDN
V
PORTN
LTC4269-1
V
PORTP
V
GND
NEG
D1
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C1
LTC4269-1
21
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