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| 型号： | LTC4257IS8#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/20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
11
4257fb
Thermal Protection
The LTC4257 includes smart thermal protection in order
to provide full device functionality in a miniature package
while maintaining safe operating temperatures. Several
factors create the possibility for tremendous power
dissipation within the LTC4257. IEEE 802.3af mandates
that inrush current be limited to less than 400mA while
standard telecom power can be as high as 57V. At turn on,
before the load capacitor has charged up, the instanta-
neous power dissipated by the LTC4257 can be over 20W.
As the load capacitor charges up, the power dissipation in
the LTC4257 will decrease until it reaches a steady-state
value dependent on the DC load current. The size of the
load capacitor determines how fast the power dissipation
in the LTC4257 subsides. At room temperature, the
LTC4257 can handle load capacitors as large as 800礔
without going into thermal shutdown. With a large load
capacitor like this, the LTC4257 die temperature will
increase by about 50癈 during a single turn-on sequence.
If for some reason power were removed from the part and
then quickly reapplied so that the LTC4257 had to charge
up the load capacitor again, the temperature rise would be
excessive if safety precautions were not implemented.
The LTC4257 protects itself from thermal damage by
monitoring the die temperature. If the die temperature
exceeds the overtemperature trip point, the part switches
to a half-power mode where the current limit is set to 50%
of its normal level. This reduces power dissipation and
helps prevent further heating. If the part continues to heat
up and reaches the shutdown temperature, the current is
reduced to zero and very little power is dissipated in the
part until it cools below the overtemperature set point. The
LTC4257 current limit will continue switching between
0%, 50% and 100% current levels (Figure 7) until the load
capacitor is fully charged.
If the PD is designed to operate at a high ambient tempera-
ture and with the maximum allowable supply (57V), there
will be a limit to the size load capacitor that can be charged
up before the LTC4257 reaches the overtemperature trip
point. Hitting the overtemperature trip point intermittently
does not harm the LTC4257, but it will delay completion of
capacitor charging. Capacitors up to 200礔 can be charged
without a problem.
During classification, excessive heating of the LTC4257
can occur if the PSE violates the 75ms probing time limit.
To protect the LTC4257, the thermal protection circuitry
will disable classification current if the die temperature
exceeds the overtemperature trip point. When the die
cools down below the trip point, classification current is
re-enabled.
Once the LTC4257 has charged up to the load capacitor
and the PD is powered and running, there will be some
residual heating due to the DC load current of the PD
flowing through the internal MOSFET. In some applica-
tions, the LTC4257 power dissipation may be significant
and if dissipated in the S8 package, excessive package
heating could occur. This problem can be solved with the
use of the DD package which has superior thermal perfor-
mance. The DD package includes an exposed pad that
should be soldered to an isolated heatsink on the printed
circuit board.
Figure 7. Smart Thermal Protection State Diagram
APPLICATIO S I FOR ATIO
U
U
U
4257 F07
T < 120癈
T < 120癈
T > 120癈
T > 140癈
100%
CURRENT
UVLO
TURN ON
50%
CURRENT
0%
CURRENT
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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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