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參數(shù)資料
| 型號(hào)： | LTC4257CS8#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 網(wǎng)絡(luò)接口 |
| 英文描述： | IEEE 802.3af PD Power over Ethernet Interface Controller; Package: SO; No of Pins: 8; Temperature Range: 0°C to +70°C |
| 中文描述： | DATACOM, NETWORK INTERFACE SUPPORT CIRCUIT, PDSO8 |
| 封裝： | 0.150 INCH, LEAD FREE, PLASTIC, SOP-8 |
| 文件頁數(shù)： | 19/20頁 |
| 文件大小： | 258K |
| 代理商： | LTC4257CS8#TRPBF |
LTC4257
8
4257fb
Measured Current and Measured Voltage. The IEEE has
since removed the Measured Voltage method from the
specification. The LTC4257 is compatible with the IEEE
802.3af standard and also works with the obsolete Mea-
sured Voltage method.
In the Measured Current method (Figure 3), the PSE
presents a fixed voltage between –15.5V and –20V to the
PD. With the input voltage in this range, the LTC4257
asserts a load current from the GND pin through the
RCLASS resistor. The magnitude of the load current is set
with the selection of the RCLASS resistor. The resistor value
associated with each class is shown in Table 2.
In the Measured Voltage method (Figure 4), the PSE drives
a current into the PD and monitors the voltage across the
PD terminals. The PSE current steps between classifica-
tion load current values in order to classify the PD under
test. For PSE probe currents below the PD load current, the
LTC4257 will keep the PD terminal voltage below the
classification voltage range. For PSE probe currents above
the PD load current, the LTC4257 will force the PD terminal
voltage above the classification voltage range.
During classification, a moderate amount of power is
dissipated in the LTC4257. IEEE 802.3af limits the classi-
fication time to 75ms. The LTC4257 is designed to handle
the power dissipation for this time period. If the PSE
Classification
Once the PSE has detected a PD, the PSE may optionally
classify the PD. Classification provides a method for more
efficient allocation of power by allowing the PSE to identify
lower-power PDs and allocate less power for these de-
vices. IEEE 802.3af defines five classes (Table 2) with
varying power levels. The designer selects the appropriate
classification based on the power consumption of the PD.
For each class, there is an associated load current that the
PD asserts onto the line during classification probing. The
PSE measures the PD load current to determine the proper
classification and PD power requirements.
Table 2. Summary of IEEE 802.3af Power Classifications and
LTC4257 RCLASS Resistor Selection
MAXIMUM
NOMINAL
LTC4257
POWER LEVELS
CLASSIFICATION
RCLASS
AT INPUT OF PD
LOAD CURRENT
RESISTOR
CLASS
USAGE
(W)
(mA)
(
, 1%)
0
Default
0.44 to 12.95
< 5
Open
1
Optional
0.44 to 3.84
10.5
124
2
Optional
3.84 to 6.49
18.5
68.1
3
Optional
6.49 to 12.95
28
45.3
4
Reserved
Reserved*
40
30.9
*Class 4 is currently reserved and should not be used.
Early revisions of the IEEE 802.3af draft specification
defined two methods that a PSE could use in order to
perform PD classification. These methods are known as
Figure 3. IEEE 802.3af Measured-Current Method
of Classification Probing
Figure 4. IEEE 802.af Measured-Voltage Method
of Classification Probing
APPLICATIO S I FOR ATIO
WU
UU
GND
RCLASS
VIN
8
2
4
LTC4257
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4257
4257 F03
RCLASS
CURRENT PATH
V
PD
PSE
PSE CURRENT MONITOR
PSE
PROBING
VOLTAGE
SOURCE
–15.5V TO –20.5V
GND
RCLASS
VIN
8
2
4
LTC4257
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4257
4257 F04
RCLASS
CURRENT PATH
V
PD
PSE
VOLTAGE
MONITOR
PSE
PROBING
CURRENT
SOURCE
IF PSE PROBING CURRENT < LTC4257 LOAD CURRENT, PD TERMINAL VOLTAGE IS < 15V
IF PSE PROBING CURRENT > LTC4257 LOAD CURRENT, PD TERMINAL VOLTAGE IS > 20V
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