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参数资料
| 型号： | DC1561B |
| 厂商： | Linear Technology |
| 文件页数： | 5/42页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD FOR LTC4278 |
| 标准包装： | 1 |
| 主要目的： | 电源管理，以太网供电（POE） |
| 嵌入式： | 否 |
| 已用 IC / 零件： | LTC4278 |
| 已供物品： | 板 |
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LTC4278
13
4278fc
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.
TheIEE802.3atstandardestablishesahigherpoweralloca-
tion for Power over Ethernet while maintaining backwards
compatibility with the existing IEEE 802.3af systems.
Powersourcingequipment(PSE)andpowereddevicesare
distinguished as Type 1 complying with the IEEE 802.3af/
IEEE 802.3at 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 classification from a PD and communi-
cating the presence of a Type 2 PSE. A Type 2 PSE has the
option of acquiring PD power classification by performing
2-event classification (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 specifically 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 classification. This
indicator bit may be used to alert the LTC4278 output load
that a Type 2 PSE is present. With an internal signature
resistor, classification 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 VPORTP and
VPORTN 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.
DETECTION V1
CLASSIFICATION
ON
OFF
POWER
BAD
OFF
ON
τ = RLOAD C1
PWRGD TRACKS
VPORTN
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
VPORTP
PWRGD
TRACKS
VPORTP
POWER
BAD
POWER
BAD
TIME
POWER
GOOD
POWER
GOOD
DETECTION I1
CLASSIFICATION
DETECTION I2
LOAD, ILOAD
4278 F01
ICLASS DEPENDENT ON RCLASS SELECTION
INRUSH = 100mA
I1 = V1 – 2 DIODE DROPS
25k
ILOAD =
VPORTP
RLOAD
I2 = V2 – 2 DIODE DROPS
25k
VPORTP
PSE
IIN
RLOAD
RCLASS
C1
RCLASS
PWRGD
LTC4278
VNEG
VPORTN
IN DETECTION
RANGE
Figure 1. VNEG, PWRGD, PWRGD and PD
Current as a Function of Input Voltage
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