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參數(shù)資料
| 型號： | LTC4259A |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Quad IEEE 802.3af Power over Ethernet Controller with AC Disconnect |
| 中文描述： | 四IEEE 802.3af標準與AC電源斷開以太網控制器 |
| 文件頁數(shù)： | 28/32頁 |
| 文件大?。?/td> | 326K |
| 代理商： | LTC4259A |
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LTC4259A
28
4259Af
APPLICATIOU
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possible pitfalls that can cause a PSE to be noncompliant.
For further assistance please contact Linear Technology’s
Applications department.
Sense Resistors
The LTC4259A is designed to use a 0.5
sense resistor,
R
S
, to monitor the current through each port. The value of
the sense resistor has been minimized in order to reduce
power loss and as a consequence, the voltage which the
LTC4259A must measure is small. Each port may be
drawing up to 450mA with this current flowing through the
sense resistor and associated circuit board traces. To
prevent parasitic resistance on the circuit board from
obscuring the voltage drop across the sense resistor, the
LTC4259A must Kelvin sense the resistor voltage. One
way to achieve Kelvin sensing is “star grounding,” shown
pictorially in Figure 1. Another option is to use a –48V
power plane to connect the sense resistor and the
LTC4259A V
EE
pin. Either of these strategies will prevent
voltages developed across parasitic circuit board resis-
tances from affecting the LTC4259A current measure-
ment accuracy. The precision of the sense resistor directly
affects the measurement of the IEEE parameters I
INRUSH
,
I
LIM
, I
CUT
and I
MIN
. Therefore, to maintain IEEE compli-
ance, use a resistor with 0.5% or better accuracy.
Power MOSFETs
The LTC4259A controls power MOSFETs in order to
regulate current flow through the Ethernet ports. Under
certain conditions these MOSFETs have to dissipate sig-
nificant power. See the Choosing External MOSFETs sec-
tion for a detailed discussion of the requirements these
devices must meet.
Common Mode Chokes
Both nonpowered and powered Ethernet connections
achieve best performance (for data transfer, power trans-
fer and EMI) when a common mode choke is used on each
port. In the name of cost reduction, some designs share a
common mode choke between two adjacent ports. Even
for nonpowered Ethernet, sharing a choke is not recom-
mended. With two ports passing through the choke, it
cannot limit the common mode current of either port.
Instead, the choke only controls the sum of both ports’
common mode current. Because cabling from the ports
generally connects to different devices up to 200m apart,
a current loop can form. In such a loop, common mode
current flows in one port and out the other, and the choke
will not prevent this because the sum of the currents is
zero. Another way to view this interaction between the
paired ports is that the choke acts as a transformer
coupling the ports’ common modes together. In
nonpowered Ethernet, common mode current results
from nonidealities like ground loops; it is not part of
normal operation. However, Power over Ethernet sends
power and hence significant current through the ports;
common mode current is a byproduct of normal opera-
tion. As described in the Choosing External MOSFETs
section and under the Power Supplies heading below,
large transients can occur when a port’s power is turned
on or off. When a powered port is shorted (see Surge
Suppressors and Circuit Protection), a port’s common
mode current may be excessive. Sharing a common mode
choke between two ports couples start-up, disconnect
and fault transients from one port to the other. The end
result can range from momentary noncompliance with
802.3af to intermittent behavior and even to excessive
voltages that may damage circuitry (in both the PSE and
PD) connected to the ports.
Detect, AC Blocking and Transient Supressor Diodes
During detection and classification, the LTC4259A senses
the port voltage through the detect diodes D
DET
in Figure
16. Excessive voltage drop across D
DET
will corrupt the
LTC4259A’s detect and classification results. Select a
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| LTC4259A-1 | 制造商:LINER 制造商全稱:Linear Technology 功能描述:Synchronous No-Opto Flyback Controller |
| LTC4259ACGW | 功能描述:IC CTRLR POE QUAD AC DISC 36SSOP RoHS:否 類別:集成電路 (IC) >> 接口 - 控制器 系列:- 標準包裝:2,450 系列:- 控制器類型:SPI 總線至 I²C 總線橋接 接口:I²C，串行，SPI 電源電壓:2.4 V ~ 3.6 V 電流 - 電源:11mA 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:24-VFQFN 裸露焊盤 供應商設備封裝:24-HVQFN（4x4） 包裝:托盤 配用:568-3511-ND - DEMO BOARD SPI TO I2C 其它名稱:935286452157SC18IS600IBSSC18IS600IBS-ND |
| LTC4259ACGW#PBF | 功能描述:IC CTRLR POE QUAD AC DISC 36SSOP RoHS:是 類別:集成電路 (IC) >> 接口 - 控制器 系列:- 標準包裝:2,450 系列:- 控制器類型:SPI 總線至 I²C 總線橋接 接口:I²C，串行，SPI 電源電壓:2.4 V ~ 3.6 V 電流 - 電源:11mA 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:24-VFQFN 裸露焊盤 供應商設備封裝:24-HVQFN（4x4） 包裝:托盤 配用:568-3511-ND - DEMO BOARD SPI TO I2C 其它名稱:935286452157SC18IS600IBSSC18IS600IBS-ND |
| LTC4259ACGW#TR | 功能描述:IC CTRLR POE QUAD AC DISC 36SSOP RoHS:否 類別:集成電路 (IC) >> 接口 - 控制器 系列:- 標準包裝:2,450 系列:- 控制器類型:SPI 總線至 I²C 總線橋接 接口:I²C，串行，SPI 電源電壓:2.4 V ~ 3.6 V 電流 - 電源:11mA 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:24-VFQFN 裸露焊盤 供應商設備封裝:24-HVQFN（4x4） 包裝:托盤 配用:568-3511-ND - DEMO BOARD SPI TO I2C 其它名稱:935286452157SC18IS600IBSSC18IS600IBS-ND |
| LTC4259ACGW#TRPBF | 功能描述:IC CTRLR POE QUAD AC DISC 36SSOP RoHS:是 類別:集成電路 (IC) >> 接口 - 控制器 系列:- 標準包裝:2,450 系列:- 控制器類型:SPI 總線至 I²C 總線橋接 接口:I²C，串行，SPI 電源電壓:2.4 V ~ 3.6 V 電流 - 電源:11mA 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:24-VFQFN 裸露焊盤 供應商設備封裝:24-HVQFN（4x4） 包裝:托盤 配用:568-3511-ND - DEMO BOARD SPI TO I2C 其它名稱:935286452157SC18IS600IBSSC18IS600IBS-ND |
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