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參數(shù)資料
| 型號： | LTC4258CGW#TRPBF |
| 廠商： | Linear Technology |
| 文件頁數(shù)： | 10/28頁 |
| 文件大小： | 0K |
| 描述： | IC POE 802.3AF QUAD 36-SSOP |
| 標(biāo)準(zhǔn)包裝： | 1,000 |
| 控制器類型： | 以太網(wǎng)供電控制器（POE） |
| 接口： | IEEE 802.3af |
| 電源電壓： | 3 V ~ 4 V |
| 電流 - 電源： | 2.5mA |
| 工作溫度： | 0°C ~ 70°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 36-BSOP（0.295"，7.50mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 36-SSOP |
| 包裝： | 帶卷 (TR) |
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LTC4258
18
4258fb
APPLICATIO S I FOR ATIO
WU
UU
Foldback
Foldback is designed to limit power dissipation in the
MOSFET during power-up and momentary short-circuit
conditions. At low port output voltages, the voltage across
the MOSFET is high, and power dissipation will be large if
significant current is flowing. Foldback monitors the port
output voltage and reduces the VLIM current limit level
linearly from its full value (212.5mV typ) at a port voltage
of 18V to approximately 1/7th of the full value (30mV typ)
at a port voltage of 0V. With 0.5
sense resistors, this
limits the short-circuit current to 60mA (typ) instead of the
full 425mA (typ) current limit. When the LTC4258 is in
foldback, the tICUT timer is active.
Short-Circuit Protection
If a port is suddenly shorted out, the MOSFET power
dissipation can rise to very high levels, jeopardizing the
MOSFET even before the normal current limit circuit can
respond. A separate short-circuit current limit circuit
watches for significant overcurrent events (VSENSE
>275mV, >550mA with a 0.5
sense resistor) and pulls
the GATE pin down immediately if such an event occurs,
shutting off the MOSFET in less than 1
s (with no external
capacitor on GATE). Approximately 100
s later, GATE is
allowed to rise back up and the normal current limit circuit
will take over, allowing ILIM current to flow and causing the
tICUT timer to count up. During a short circuit, ILIM will be
reduced by the foldback feature to 1/7th of the nominal
value.
Figures 14 and 15 show the LTC4258 controlling port
current during short circuits. In Figure 14, the MOSFET is
turned off 0.5
s after the port is shorted with 1. The
spike in port voltage and current at the moment the
MOSFET turns off is the response of inductance in the
system, such as the magnetics and the Ethernet cable; see
Surge Suppressors and Circuit Protection for further
details. The 0.1
F port bypass cap (see Figure 1) provides
some port current for 0.25
s after the MOSFET is off. In
Figure 15, the LTC4258 quickly turns the port off and the
spike above ground is again due to inductance. It then
ramps the MOSFET gate up, similar to applying power
after a PD is detected, bringing the port into a controlled
425mA (typ) ILIM current limit. When the short is removed,
the port current no longer needs to be limited and LTC4258
ramps up its GATE pin to fully enhance the MOSFET.
Short-circuit protection quickly stops excessive current
and limits the energy delivered to a short or faulty PD. Yet
the LTC4258 only stops current briefly, so momentary
faults typically do not cause the PD to lose power and PDs
receive at least 50ms of 400mA to 450mA peak current as
required by the 802.3af standard.
Figure 14. Rapid Response to 1
Short
Figure 15. Rapid Response to Momentary 100
Short
250ns/DIV
4258 G04
GND
PORT
VOLTAGE
20V/DIV
GATE
VOLTAGE
10V/DIV
PORT
CURRENT
20A/DIV
VEE
+15V
VDD = 3.3V
VEE = –48V
VEE
0mA
SHORT APPLIED
FET OFF
FAST PULL-DOWN
ACTIVATED
100
s/DIV
4258 G05
PORT
VOLTAGE
20V/DIV
GATE
VOLTAGE
10V/DIV
PORT
CURRENT
500mA/DIV
VDD = 3.3V
VEE = –48V
VEE
GND
0mA
CURRENT LIMIT
SHORT APPLIED
SHORT REMOVED
FAST PULL-DOWN
VEE
+15V
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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 |
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