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參數(shù)資料
| 型號： | LTC4257 |
| 廠商： | Linear Technology Corporation |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 抗輻射高效，5安培開關穩(wěn)壓器 |
| 文件頁數(shù)： | 12/32頁 |
| 文件大?。?/td> | 1975K |
| 代理商： | LTC4257 |
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LTC4267
12
4267f
V
PORTP
V
PORTN
LTC4267
4267 F03
25k SIGNATURE
RESISTOR
SIGNATURE DISABLE
SIGDISA
9k
16k
TO
PSE
Figure 3. 25k Signature Resistor with Disable
Classification
Once the PSE has detected a PD, the PSE may option-
ally 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 devices. The IEEE 802.3af specification 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 as-
sociated 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.
During classification (Figure 4), the PSE presents a fixed
voltage between –15.5V and –20.5V to the PD. With the
input voltage in this range, the LTC4267 asserts a load
current from the V
PORTP
pin through the R
CLASS
resistor.
The magnitude of the load current is set by the R
CLASS
resistor. The resistor values associated with each class
are shown in Table 2. Note that the switching regulator
will not interfere with the classification measurement since
the LTC4267 has not passed power to the regulator.
Table 2. Summary of IEEE 802.3af Power Classifications and
LTC4267 R
CLASS
Resistor Selection
Maximum
Power Levels
at Input of PD
Class
Usage
(W)
0
Default
0.44 to 12.95
1
Optional
0.44 to 3.84
2
Optional
3.84 to 6.49
3
Optional
6.49 to 12.95
4
Reserved
Reserved*
*Class 4 is currently reserved and should not be used.
Nominal
Classification
Load Current
(mA)
<5
10.5
18.5
28
40
LTC4267
R
CLASS
Resistor
(
Ω
, 1%)
Open
124
68.1
45.3
30.9
The IEEE 802.3af specification limits the classification
time to 75ms because a significant amount of power is
dissipated in the PD. The LTC4267 is designed to handle the
power dissipation for this time period. If the PSE probing
exceeds 75ms, the LTC4267 may overheat. In this situation,
the thermal protection circuit will engage and disable the
classification current source in order to protect the part.
The LTC4267 stays in classification mode until the input
voltage rises above the UVLO turn-on voltage.
V
PORTN
Undervoltage Lockout
The IEEE specification dictates a maximum turn-on voltage
of 42V and a minimum turn-off voltage of 30V for the PD.
In addition, the PD must maintain large on-off hysteresis
to prevent resistive losses in the wiring between the PSE
and the PD from causing start-up oscillation. The LTC4267
incorporates an undervoltage lockout (UVLO) circuit that
monitors the line voltage at V
PORTN
to determine when
to apply power to the integrated switching regulator
(Figure 5). Before the power is applied to the switching
regulator, the P
OUT
pin is high impedance and sitting at
the ground potential since there is no charge on capacitor
C1. When the input voltage rises above the UVLO turn-on
threshold, the LTC4267 removes the detection and clas-
sification loads and turns on the internal power MOSFET.
C1 charges up under the LTC4267 current limit control
and the P
OUT
pin transitions from 0V to V
PORTN
. This
sequence is shown in Figure 1. The LTC4267 includes
a hysteretic UVLO circuit on V
PORTN
that keeps power
applied to the load until the input voltage falls below the
UVLO turn-off threshold. Once the input voltage drops
below –30V, the internal power MOSFET is turned off and
Figure 4. IEEE 802.3af Classification Probing
APPLICATIOU
W
U
U
V
PORTP
R
CLASS
V
PORTN
LTC4267
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4267
4267 F04
R
CLASS
CURRENT PATH
V
PD
PSE
PSE CURRENT MONITOR
PSE
PROBING
VOLTAGE
SOURCE
–15.5V TO –20.5V
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相關代理商/技術參數(shù) |
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| LTC4257-1 | 制造商:LINER 制造商全稱:Linear Technology 功能描述:Power over Ethernet Interface Controller with Dual Current Limit |
| LTC4257CDD | 功能描述:IC CONTROLLER POE INTERFACE 8DFN RoHS:否 類別:集成電路 (IC) >> PMIC - 熱交換 系列:- 標準包裝:50 系列:- 類型:熱交換控制器 應用:-48V 遠程電力系統(tǒng)，AdvancedTCA ? 系統(tǒng)，高可用性 內部開關:無 電流限制:可調 電源電壓:11.5 V ~ 14.5 V 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:10-TFSOP，10-MSOP（0.118"，3.00mm 寬） 供應商設備封裝:10-MSOP 包裝:管件 |
| LTC4257CDD#PBF | 功能描述:IC CONTROLLER POE INTERFACE 8DFN RoHS:是 類別:集成電路 (IC) >> PMIC - 熱交換 系列:- 標準包裝:50 系列:- 類型:熱交換控制器 應用:-48V 遠程電力系統(tǒng)，AdvancedTCA ? 系統(tǒng)，高可用性 內部開關:無 電流限制:可調 電源電壓:11.5 V ~ 14.5 V 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:10-TFSOP，10-MSOP（0.118"，3.00mm 寬） 供應商設備封裝:10-MSOP 包裝:管件 |
| LTC4257CDD#TR | 功能描述:IC CONTROLLER POE INTERFACE 8DFN RoHS:否 類別:集成電路 (IC) >> PMIC - 熱交換 系列:- 標準包裝:50 系列:- 類型:熱交換控制器 應用:-48V 遠程電力系統(tǒng)，AdvancedTCA ? 系統(tǒng)，高可用性 內部開關:無 電流限制:可調 電源電壓:11.5 V ~ 14.5 V 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:10-TFSOP，10-MSOP（0.118"，3.00mm 寬） 供應商設備封裝:10-MSOP 包裝:管件 |
| LTC4257CDD#TRPBF | 功能描述:IC CONTROLLER POE INTERFACE 8DFN RoHS:是 類別:集成電路 (IC) >> PMIC - 熱交換 系列:- 標準包裝:50 系列:- 類型:熱交換控制器 應用:-48V 遠程電力系統(tǒng)，AdvancedTCA ? 系統(tǒng)，高可用性 內部開關:無 電流限制:可調 電源電壓:11.5 V ~ 14.5 V 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:10-TFSOP，10-MSOP（0.118"，3.00mm 寬） 供應商設備封裝:10-MSOP 包裝:管件 |
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