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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(biāo)準(zhǔn)與AC電源斷開以太網(wǎng)控制器 |
| 文件頁數(shù)： | 29/32頁 |
| 文件大小： | 326K |
| 代理商： | LTC4259A |
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LTC4259A
29
4259Af
diode for D
DET
that will have less than 0.7V of forward drop
at 0.4mA and less than 0.9V of forward drop at 50mA.
When the port is powered, the detect diode is reverse bi-
ased. Any leakage through the detect diode prevents the
LTC4259A from sensing all the current coupled through the
C
DET
capacitor. At high temperature with 70V of reverse
bias, a typical switching diode like the 1N4148 may have
more than 50
μ
A of leakage. Such leakage can interfere with
AC disconnect because it is a large fraction of the
LTC4259A’s I
ACDMIN
threshold. Using a low leakage detect
diode like the CMPD3003 is recommended.
The AC blocking diodes can interfere with AC disconnect
sensing if they become leaky. If the AC blocking diode (D
AC
in Figure 16) begins leaking, it contributes to the Ethernet
port impedance, potentially bringing the impedance low
enough to draw I
ACDMIN
from the DETECT pin and keep the
port powered. More likely, leakage through the AC block-
ing diode will cause shifts in the AC disconnect threshold
that are not large enough to make the PSE noncompliant.
Generally, diode leakage is caused by voltage or tempera-
ture stress. Diodes that are rated to 100V or more and can
handle dissipating at least 0.5W should be acceptable in
this application. Other component leakages can have a
similar affect on AC disconnect and even affect DC discon-
nect if the leakage becomes severe. Among components
to be wary of are the transient surge suppressors. The
devices shown in Figure 1 are rated for less than 5
μ
A of
leakage at 58V. However there is a potential for stress
induced leakage, so healthy margins should be used when
selecting diodes for these applications.
Capacitors
Sizing of both the C
DET
and C
PSE
capacitors is critical to
proper operation of the LTC4259A’s AC disconnect sens-
APPLICATIOU
W
U
U
ing. See the AC Disconnect section for more information.
Also, C
PSE
may be important to the voltage stability of a
powered port. Port voltage instability is generally not a
problem if V
EE
, the –48V supply, is well bypassed. For both
of these reasons be aware that many ceramic dielectrics
have dramatic DC voltage and temperature coefficients. A
0.22
μ
F ceramic capacitor is often nowhere near 0.22
μ
F
when operating at 50VDC or 100VDC. Use 100V or higher
rated X7R capacitors for C
DET
and C
PSE
as these have re-
duced voltage dependance while also being relatively small
and inexpensive.
Power Supplies
The LTC4259A must be supplied with 3.3V (V
DD
) and
–48V (V
EE
). Poor regulation on either of these supplies
can lead to noncompliance. The IEEE requires a PSE
output voltage between 44V and 57V. When the LTC4259A
begins powering an Ethernet port, it controls the current
through the port to minimize disturbances on V
EE
. How-
ever, if the V
EE
supply is underdamped or otherwise
unstable, its voltage could go outside of the IEEE specified
limits, causing all ports in the PSE to be noncompliant.
This scenario can be even worse when a PD is unplugged
because the current can drop immediately to zero. In both
cases the port voltage must always stay between –44V
and –57V. In addition, the 802.3af specification places
specific ripple, noise and load regulation requirements on
the PSE. Among other things, disturbances on either V
DD
or V
EE
can adversely affect detection, classification and the
AC disconnection sensing. Proper bypassing and stability
of the V
DD
and V
EE
supplies is important.
Another problem that can affect the V
EE
supply is insuffi-
cient power, leading to the supply voltage drooping out of
the specified range. The 802.3af specification states that
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| 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(biāo)準(zhǔn)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(yīng)商設(shè)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(biāo)準(zhǔn)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(yīng)商設(shè)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(biāo)準(zhǔn)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(yīng)商設(shè)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(biāo)準(zhǔn)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(yīng)商設(shè)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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