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參數(shù)資料
型號(hào)： DC1561B
廠商： Linear Technology
文件頁數(shù)： 6/42頁
文件大?。?/td> 0K
描述： EVAL BOARD FOR LTC4278
標(biāo)準(zhǔn)包裝： 1
主要目的： 電源管理，以太網(wǎng)供電（POE）
嵌入式： 否
已用 IC / 零件： LTC4278
已供物品： 板
LTC4278
14
4278fc
APPLICATIONS INFORMATION
Table 1. LTC4278 Modes of Operation as a Function
of Input Voltage
VPORTP–VPORTN (V) LTC4278 MODES OF OPERATION
0V to 1.4V
Inactive (Reset After 1st Classification Event)
1.5V to 9.8V
(5.4V to 9.8V)
25k Signature Resistor Detection Before 1st
Classification Event (Mark, 11k Signature
Corrupt After 1st Classification Event)
12.5V to ON/OFF*
Classification Load Current Active
ON/OFF* to 60V
Inrush and Power Applied To PD Load
>71V
Overvoltage Lockout,
Classification and Hot Swap Are Disabled
*ON/OFF includes hysteresis. Rising input threshold, 37.2V Max.
Falling input threshold, 30V Min.
These modes satisfy the requirements defined in the
IEEE 802.3af/IEEE 802.3at specification.
INPUT DIODE BRIDGE
In the IEEE 802.3af/IEEE 802.3at standard, the modes of
operation reference the input voltage at the PD’s RJ45
connector. Since the PD must handle power received in
either polarity from either the data or the spare pair, input
diode bridges BR1 and BR2 are connected between the
RJ45 connector and the LTC4278 (Figure 2).
The input diode bridge introduces a voltage drop that
affects the range for each mode of operation. The
LTC4278 compensates for these voltage drops so that a
PD built with the LTC4278 meets the IEEE 802.3af/IEEE
802.3at-established voltage ranges. Note the Electrical
CharacteristicsarereferencedwithrespecttotheLTC4278
package pins.
DETECTION
During detection, the PSE looks for a 25k signature resis-
tor which identifies the device as a PD. The PSE will apply
two voltages in the range of 2.8V to 10V and measures
the corresponding currents. Figure 1 shows the detection
voltages V1 and V2 and the corresponding PD current.
The PSE calculates the signature resistance using the
ΔV/
ΔI measurement technique.
TheLTC4278presentsitsprecision,temperature-compen-
sated 25k resistor between the VPORTP and VPORTN pins,
alerting the PSE that a PD is present and requests power
to be applied. The LTC4278 signature resistor also com-
pensates for the additional series resistance introduced by
the input diode bridge. Thus a PD built with the LTC4278
conforms to the IEEE 802.3af/IEEE 802.3at specifications.
RX–
6
RX+
3
TX–
2
TX+
RJ45
T1
POWERED
DEVICE
(PD)
INPUT
4278 F02
1
7
8
5
4
SPARE–
SPARE+
TO PHY
BR2
0.1F
100V
BR1
D3
LTC4278
VPORTN
VPORTP
Figure 2. PD Front End Using Diode Bridges on Main and Spare Inputs
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