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参数资料
| 型号： | LTC4257IS8#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 9/20页 |
| 文件大小： | 263K |
| 描述： | IC CTRLR INTFACE PWR/ETHER 8SOIC |
| 产品培训模块： | Power over Ethernet |
| 标准包装： | 100 |
| 类型： | 以太网供电开关（PoE） |
| 应用： | IP 电话，LAN 网络供电，网络路由器和交换机 |
| 内部开关： | 是 |
| 电流限制： | 350mA |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 8-SOIC |
| 包装： | 管件 |
| 产品目录页面： | 1338 (CN2011-ZH PDF) |
LTC4257
9
4257fb
probing exceeds 75ms, the LTC4257 may overheat. In this
situation, the thermal protection circuit will engage and
disable the classification current source in order to protect
the part. The LTC4257 stays in classification mode until
the input voltage rises above the UVLO turn-on voltage.
Undervoltage Lockout
IEEE 802.3af 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 LTC4257
incorporates an undervoltage lockout (UVLO) circuit that
monitors line voltage to determine when to apply power
to the PD load (Figure 5). Before power is applied to the
load, the V
OUT
pin is high impedance and at ground
potential since there is no charge on capacitor C1. When
the input voltage rises above the UVLO turn-on threshold,
the LTC4257 removes the classification load current and
turns on the internal power MOSFET. C1 charges up under
LTC4257 current limit control and the V
OUT
pin transitions
from 0V to V
IN
. This sequence is shown in Figure 1. The
LTC4257 includes a hysteretic UVLO circuit 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 the classification load current is re-enabled.
C1 will discharge through the PD circuitry and the V
OUT
pin will go to a high impedance state.
Input Current Limit
IEEE 802.3af specifies a maximum inrush current and also
specifies a minimum load capacitor between the GND and
V
OUT
pins. To control turn-on surge current in the system,
the LTC4257 integrates a current limit circuit with the
onboard power MOSFET and sense resistor to provide a
complete inrush control circuit without additional external
components. The LTC4257 limits input current to less
than the 400mA maximum specified by 802.3af, allowing
the load capacitor to ramp up to the line voltage in a
controlled manner. During this ramp up, a large amount of
power is dissipated in the power MOSFET. The LTC4257
Figure 5. LTC4257 Undervoltage Lockout
APPLICATIO S I FOR ATIO
U
U
U
GND
C1
5礔
MIN
V
IN
8
4
V
OUT
5
LTC4257
4257 F05
TO
PSE
UNDERVOLTAGE
LOCKOUT
CIRCUIT
PD
LOAD
CURRENT-LIMITED
TURN ON
+
INPUT
LTC4257
VOLTAGE
POWER MOSFET
0V TO UVLO*
OFF
>UVLO*
ON
*UVLO INCLUDES HYSTERESIS
RISING INPUT THRESHOLD E 39.2V
FALLING INPUT THRESHOLD E 30.5V
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LTC4258 | 制造商:LINER 制造商全称:Linear Technology 功能描述:Quad IEEE 802.3at Power over Ethernet Controller |
| LTC4258_09 | 制造商:LINER 制造商全称:Linear Technology 功能描述:Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection |
| LTC4258CGW | 制造商:Linear Technology 功能描述:POWER OVER ETHERNET CONTROLLER |
| LTC4258CGW#PBF | 功能描述:IC POE 802.3AF QUAD 36-SSOP RoHS:是 类别:集成电路 (IC) >> 接口 - 控制器 系列:- 标准包装:2,450 系列:- 控制器类型:SPI 总线至 I²C 总线桥接 接口:I²C，串行，SPI 电源电压:2.4 V ~ 3.6 V 电流 - 电源:11mA 工作温度:-40°C ~ 85°C 安装类型:表面贴装 封装/外壳:24-VFQFN 裸露焊盘 供应商设备封装:24-HVQFN（4x4） 包装:托盘 配用:568-3511-ND - DEMO BOARD SPI TO I2C 其它名称:935286452157SC18IS600IBSSC18IS600IBS-ND |
| LTC4258CGW#TR | 制造商:Linear Technology 功能描述:POWER OVER ETHERNET CONTROLLER |
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