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參數(shù)資料
| 型號(hào)： | 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)： | 18/32頁 |
| 文件大小： | 326K |
| 代理商： | LTC4259A |
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LTC4259A
18
4259Af
APPLICATIOU
W
U
U
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 V
LIM
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 LTC4259A is in
foldback, the t
ICUT
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 (V
SENSE
>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
μ
s (with no external
capacitor on GATE). Approximately 100
μ
s later, GATE is
allowed to rise back up and the normal current limit circuit
will take over, allowing I
LIM
current to flow and causing the
t
ICUT
timer to count up. During a short circuit, I
LIM
will be
reduced by the foldback feature to 1/7th of the nominal
value.
Choosing External MOSFETs
Power delivery to the ports is regulated with external
power MOSFETs. These MOSFETs are controlled as previ-
ously described to meet the IEEE 802.3af specification.
Under normal operation, once the port is powered and the
PD’s bypass capacitor is charged to the port voltage, the
external MOSFET dissipates very little power. This sug-
gests that a small MOSFET is adequate for the job. Unfor-
tunately, other requirements of the IEEE 802.3af mandate
a MOSFET capable of dissipating significant power. When
the port is being powered up, the port voltage must reach
30V or more before the PD turns on. The port voltage can
then drop to 0V as the PD’s bypass capacitor is charged.
According to the IEEE, the PD can directly connect a 180
μ
F
capacitor to the port and the PSE must charge that
capacitor with a current limit of 400mA to 450mA for at
least 50ms.
An even more extreme example is a noncompliant PD that
provides the proper signature during detection but then
behaves like a low valued resistor, say 50
, in parallel with
a 1
μ
F capacitor. When the PSE has charged this
noncompliant PD up to 20V, the 50
resistor will draw
400mA (the minimum IEEE prescribed I
LIM
current limit)
keeping the port voltage at 20V for the remainder of t
START
.
The external MOSFET sees 24V to 37V V
DS
at 400mA to
450mA, dissipating 9.6W to 16.7W for 60ms (typ).
The LTC4259A implements foldback to reduce the current
limit when the MOSFET V
DS
is high; see the Foldback
section. Without foldback, the MOSFET could see as much
as 25.7W for 60ms (typ) when powering a shorted or a
noncompliant PD with only a few ohms of resistance. With
foldback, the MOSFET sees a maximum of 18W for the
duration of t
START
.
The LTC4259A’s duty cycle protection enforces 15 times
longer off time than on time, preventing successive at-
tempts to power a defective PD from damaging the
MOSFET. System software can enforce even longer wait
times. When the LTC4259A is operated in semiauto or
manual mode—described in more detail under Operating
Modes—it will not power on a port until commanded to do
so by the host controller. By keeping track of t
START
and
t
ICUT
faults, the host controller can delay turning on the
port again after one of these faults even if the LTC4259A
reports a Detect Good. In this way the host controller
implements a MOSFET cooling off period which may be
programmed to protect smaller MOSFETs from repeated
thermal cycling. The LTC4259A has built-in duty cycle
protection for t
ICUT
and t
START
(see t
ICUT
Timing and t
START
Timing sections) that is sufficient to protect the MOSFETs
shown in Figure 1.
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相關(guān)代理商/技術(shù)參數(shù) |
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|---|---|
| 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(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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