ltc4269cdkd-2 Linear Technology Corporation, ltc4269cdkd-2 Datasheet - Page 13

• Manufacturer Part Number: ltc4269cdkd-2
• Description: Ieee 802.3at High Power Pd And Synchronous Forward Controller With Aux Support
• Manufacturer: Linear Technology Corporation

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 12.95W 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 LTC4269-2 is specifi cally designed to support a PD
that must operate under the IEEE 802.3at standard. In
particular, the LTC4269-2 provides the T2P indicator bit
which recognizes 2-event classifi cation. This indicator
bit may be used to alert the LTC4269-2 output load that
a Type 2 PSE is present. With an internal signature resis-
tor, classifi cation circuitry, inrush control, and thermal
shutdown, the LTC4269-2 is a complete PD interface
solution capable of supporting in the next generation PD
applications. In addition to the PD front end, the LTC4269-2
also incorporates a high effi ciency synchronous forward
controller that minimizes component sizes while maximiz-
ing output power.
MODES OF OPERATION
The LTC4269-2 has several modes of operation depend-
ing on the input voltage applied between the V
V
PORTN
pins. Figure 1 presents an illustration of voltage
PORTP
and
PSE
Figure 1. Output Voltage, PWRGD, PWRGD and
PD Current as a Function of Input Voltage
R
I
INRUSH = 100mA
I
I
1
CLASS
LOAD
–10
–20
–30
–40
–50
CLASS
I
50
40
30
20
10
50
40
30
20
10
20
10
=
IN
V1 – 2 DIODE DROPS
=
DEPENDENT ON R
V
R
PORTP
LOAD
UVLO
INRUSH
dV
dt
R
V
DETECTION V1
25kΩ
CLASS
PORTN
=
PWRGD
TRACKS
POWER
POWER
V
LTC4269-2
INRUSH
BAD
PORTP
BAD
DETECTION V2
DETECTION I
DETECTION I
CLASSIFICATION
CLASSIFICATION
C1
PWRGD
PWRGD
V
PORTP
I
V
CLASS
ON
NEG
CLASS
POWER
POWER
ON
GOOD
GOOD
I
2
1
2
=
SELECTION
V2 – 2 DIODE DROPS
LTC4269-2
PWRGD TRACKS
V
UVLO
POWER
PORTN
25kΩ
LOAD, I
= R
TRACKS
BAD
POWER
PWRGD
42692 F01
V
UVLO
PORTP
BAD
C1
LOAD
TIME
TIME
TIME
TIME
TIME
IN DETECTION
RANGE
LOAD
R
C1
LOAD
13
42692f