LTC4278CDKD#PBF Linear Technology, LTC4278CDKD#PBF Datasheet - Page 13

• Manufacturer Part Number: LTC4278CDKD#PBF
• Description: IC PD IEEE 802.3AT 25.5W 32-DFN
• Manufacturer: Linear Technology

Specifications of LTC4278CDKD#PBF

• Controller Type: Powered Device Interface Controller (PD)
• Interface: IEEE 802.3af
• Voltage - Supply: 4.5 V ~ 20 V
• Current - Supply: 6.4mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 32-DFN
• Input Voltage: 60V
• Supply Current: 1.35mA
• Digital Ic Case Style: DFN
• No. Of Pins: 32
• Duty Cycle (%): 88%
• Frequency: 100kHz
• Operating Temperature Range: °C To +70°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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/IEEE 802.3at power levels, or Type 2 com-
plying 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 LTC4278 is specifi cally designed to support the front
end of a PD that must operate under the IEEE 802.3at
standard. In particular, the LTC4278 provides the T2P
indicator bit which recognizes 2-event classifi cation. This
indicator bit may be used to alert the LTC4278 output load
that a Type 2 PSE is present. With an internal signature
resistor, classifi cation circuitry, inrush control, and ther-
mal shutdown, the LTC4278 is a complete PD Interface
solution capable of supporting in the next generation PD
applications.
MODES OF OPERATION
The LTC4278 has several modes of operation depending on
the input voltage applied between the V
pins. Figure 1 presents an illustration of voltage and current
waveforms the LTC4278 may encounter with the various
modes of operation summarized in Table 1.
PORTP
and V
PORTN
PSE
Figure 1. V
Current as a Function of Input Voltage
I
I
INRUSH = 100mA
I
1
CLASS
LOAD
R
–10
–20
–30
–40
–50
50
40
30
20
10
50
40
30
20
10
20
10
CLASS
=
I
IN
V1 – 2 DIODE DROPS
=
DEPENDENT ON R
V
R
PORTP
LOAD
NEG
dV
INRUSH
dt
OFF
DETECTION V1
25kΩ
R
V
=
TRACKS
POWER
PWRGD
POWER
V
CLASS
PORTN
, PWRGD, PWRGD and PD
INRUSH
PORTP
BAD
BAD
DETECTION V2
DETECTION I
DETECTION I
CLASSIFICATION
CLASSIFICATION
LTC4278
C1
PWRGD
PWRGD
V
ON
CLASS
PORTP
V
NEG
POWER
POWER
ON
GOOD
GOOD
I
2
1
2
=
SELECTION
V2 – 2 DIODE DROPS
PWRGD TRACKS
V
OFF
τ = R
POWER
PORTN
25kΩ
LOAD, I
TRACKS
BAD
PWRGD
POWER
V
PORTP
BAD
OFF
LTC4278
LOAD
TIME
TIME
TIME
TIME
TIME
R
IN DETECTION
RANGE
LOAD
LOAD
C1
C1
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