Datasheet

LTC4269-1
12
42691fc
APPLICATIONS INFORMATION
OVERVIEW
Power over Ethernet (PoE) continues to gain popularity as
more products are taking advantage of having DC power
and high speed data available from a single RJ45 connector.
As PoE continues to grow in the marketplace, Powered
Device (PD) equipment vendors are running into the 13.0W
power limit established by the IEEE 802.3af standard.
The IEE802.3at standard establishes a higher power
allocation for Power over Ethernet while maintaining
backwards compatibility with the existing IEEE 802.3af
systems. Power sourcing equipment (PSE) and powered
devices are distinguished as Type 1 complying with the
IEEE 802.3af power levels, or Type 2 complying with the
IEEE 802.3at power levels. The maximum available power
of a Type 2 PD is 25.5W.
The IEEE 802.3at standard also establishes a new method
of acquiring power classifi cation from a PD and communi-
cating the presence of a Type 2 PSE. A Type 2 PSE has the
option of acquiring PD power classifi cation by performing
2-event classifi cation (layer 1) or by communicating with
the PD over the data line (layer 2). In turn, a Type 2 PD
must be able to recognize both layers of communications
and identify a Type 2 PSE.
The LTC4269-1 is specifi cally designed to support the
front end of a PD that must operate under the IEEE 802.3at
standard. In particular, the LTC4269-1 provides the T2P
indicator bit which recognizes 2-event classifi cation.
This indicator bit may be used to alert the LTC4269-1
output load that a Type 2 PSE is present. With an internal
signature resistor, classifi cation circuitry, inrush control,
and thermal shutdown, the LTC4269-1 is a complete PD
Interface solution capable of supporting in the next gen-
eration PD applications.
MODES OF OPERATION
The LTC4269-1 has several modes of operation depend-
ing on the input voltage applied between the V
PORTP
and
V
PORTN
pins. Figure 1 presents an illustration of voltage
and current waveforms the LTC4269-1 may encounter with
the various modes of operation summarized in Table 1.
DETECTION V1
CLASSIFICATION
ON
OFF
OFF
POWER
BAD
OFFON
T = R
LOAD
C1
PWRGD TRACKS
V
PORTN
DETECTION V2
50
TIME
40
30
V
PORTP
(V)
20
10
50
40
30
20
10
TIME
V
PORTP
– V
NEG
(V)
–10
TIME
–20
–30
V
PORTP
– PWRGD (V)PWRGD – V
NEG
(V)
–40
–50
20
10
PD CURRENT
INRUSH
dV
dt
INRUSH
C1
=
POWER
BAD
PWRGD
TRACKS
V
PORTP
PWRGD
TRACKS
V
PORTP
POWER
BAD
POWER
BAD
TIME
TIME
POWER
GOOD
POWER
GOOD
DETECTION I
1
CLASSIFICATION
DETECTION I
2
LOAD, I
LOAD
42691 F01
I
CLASS
DEPENDENT ON R
CLASS
SELECTION
INRUSH = 100mA
I
1
=
V1 – 2 DIODE DROPS
25kΩ
I
LOAD
=
V
PORTP
R
LOAD
I
2
=
V2 – 2 DIODE DROPS
25kΩ
V
PORTP
PSE
I
IN
R
LOAD
R
CLASS
C1
R
CLASS
PWRGD
PWRGD
LTC4269-1
V
NEG
V
PORTN
IN DETECTION
RANGE
Figure 1. V
NEG
, PWRGD, PWRGD and PD
Current as a Function of Input Voltage