LTC4265 LINER [Linear Technology], LTC4265 Datasheet - Page 6

• Manufacturer Part Number: LTC4265
• Description: IEEE 802.3at High Power PD Interface Controller with 2-Event Classifi cation Recognition
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC4265
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 12.95W power limit established by the IEEE 802.3af
standard.
The IEE802.3at standard establishes a higher power alloca-
tion for Power-over-Ethernet while maintaining backwards
compatibility with the existing IEEE802.3af systems. Power
Sourcing Equipments (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 IEEE802.3at standard also establishes a new method of
acquiring power classifi cation from a PD and communicat-
ing 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 LTC4265 is specifi cally designed to support the front
end of a PD that must operate under the IEEE802.3at
standard. In particular, the LTC4265 provides the T2PSE
indicator bit which recognizes 2-event classifi cation. This
indicator bit may be used to alert the LTC4265 output load
that a Type-2 PSE is present. With an internal signature
resistor, classifi cation circuitry, inrush control, and ther-
mal shutdown, the LTC4265 is a complete PD Interface
solution capable of supporting in the next generation PD
applications.
MODES OF OPERATION
The LTC4265 has several modes of operation depending on
the input voltage applied between the GND and V
Figure 1 presents an illustration of voltage and current
waveforms the LTC4265 may encounter with the various
modes of operation summarized in Table 1.
6
IN
pins.
PSE
Figure 1. Output Voltage, PWRGD, PWRGD and
PD Current as a Function of Input Voltage
R
I
I
INRUSH = 100mA
I
1
CLASS
–10
–20
–30
–40
–50
LOAD
CLASS
I
50
40
30
20
10
50
40
30
20
10
20
10
=
IN
V1 – 2 DIODE DROPS
=
DEPENDENT ON R
R
V
LOAD
UVLO
INRUSH
IN
dV
dt
DETECTION V1
25kΩ
R
V
=
PWRGD
TRACKS
POWER
POWER
CLASS
IN
LTC4265
INRUSH
BAD
GND
BAD
DETECTION V2
DETECTION I
DETECTION I
CLASSIFICATION
CLASSIFICATION
PWRGD
PWRGD
C1
V
GND
OUT
ON
CLASS
POWER
POWER
ON
I
GOOD
GOOD
2
2
1
=
SELECTION
V2 – 2 DIODE DROPS
PWRGD TRACKS
V
UVLO
POWER
IN
25kΩ
LOAD, I
TRACKS
BAD
= R
POWER
PWRGD
R
UVLO
LOAD
GND
BAD
C1
LOAD
TIME
TIME
TIME
TIME
TIME
IN DETECTION
RANGE
LOAD
C1
4265 F01
V
OUT
GND
4265f