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參數(shù)資料
| 型號： | LTC4263IS#TRPBF |
| 廠商： | Linear Technology |
| 文件頁數(shù)： | 14/24頁 |
| 文件大?。?/td> | 354K |
| 描述： | IC IEEE 803.2AF CNTRLR 14-SOIC |
| 標(biāo)準(zhǔn)包裝： | 2,500 |
| 類型： | 以太網(wǎng)供電（PoE） |
| 應(yīng)用： | 用于以太網(wǎng)供電（PoE）設(shè)備的電源接口開關(guān) |
| 電源電壓： | 48V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 14-SOIC（0.154"，3.90mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 14-SOIC |
| 包裝： | 帶卷 (TR) |
LTC4263
14
4263fe
APPLICATIONS INFORMATION
Power Control
The primary function of the LTC4263 is to control the
delivery of power to the PSE port. In order to meet IEEE
802.3af requirements and provide a robust solution, a
variety of current limit and current monitoring functions
are needed, as shown in Figure 6. All control circuitry
is integrated and the LTC4263 requires no external
MOSFET, sense resistor, or microcontroller to achieve
IEEE compliance.
The LTC4263 includes an internal MOSFET for driving
the PSE port. The LTC4263 drives the gate of the internal
MOSFET while monitoring the current and the output volt-
age at the OUT pin. This circuitry couples the 48V input
supply to the port in a controlled manner that satis es
the PDs power needs while minimizing disturbances on
the 48V backplane.
Figure 6. Current Thresholds and Current Limits
Port Overload
A PSE port is permitted to supply up to 15.4W continuously
and up to 400mA (I
CUT
) for up to 75ms (t
OVLD
) when in
overload. Per the IEEE 802.3af speci cation, the PSE is
required to remove power if a port stays in an overload
condition. The LTC4263 monitors port current and removes
port power if port current exceeds 375mA (typ) for greater
than 62ms (typ).
Port Inrush and Short-Circuit
The IEEE 802.3af standard lists two separate maximum
current limits, I
INRUSH
and I
LIM
, that a PSE must implement.
I
INRUSH
refers to current at port turn-on and I
LIM
is the
maximum allowable current in the case of a short after the
port is powered. Because the IEEE speci cation calls out
identical values, the LTC4263 implements both as a single
current limit referred to as I
LIM
.
When 48V power is applied to the port, the LTC4263 is
designed to power-up the PD in a controlled manner without
causing transients on the input supply. To accomplish this,
the LTC4263 implements inrush current limit. At turn-on,
current limit will allow the port voltage to quickly rise
until the PD reaches its input turn-on threshold. At this
point, the PD begins to draw current to charge its bypass
capacitance, slowing the rate of port voltage increase.
If at any time the port is shorted or an excessive load is
applied, the LTC4263 limits port current to avoid a haz-
ardous condition. The current is limited to I
LIM
for port
voltages above 30V and is reduced for lower port voltages
(see the Foldback section). Inrush and short-circuit cur-
rent limit are allowed to be active for 62ms (typ) before
the port is shut off.
Port Fault
If the port is suddenly shorted, the internal MOSFET power
dissipation can rise to very high levels until the short-circuit
current limit circuit can respond. A separate high speed
current limit circuit detects severe fault conditions (I
OUT
>
650mA (typ)) and quickly turns off the internal MOSFET if
such an event occurs. The circuit then limits current to I
LIM
while the t
OVLD
timer increments. During a short-circuit,
I
LIM
will be reduced by the foldback circuitry.
t
OVLD
Timing
For overload, inrush, and short-circuit conditions, the
IEEE 802.3af standard limits the duration of these events
to 50ms-75ms. The LTC4263 includes a 62ms (typ) t
OVLD
timer to monitor overload conditions. The timer is incre-
mented whenever current greater than I
CUT
ows through
the port. If the current is still above I
CUT
when the t
OVLD
timer expires, the LTC4263 will turn off power to the port
and ash the LED. In this situation, the LTC4263 waits
four seconds and then restarts detection. If the overload
0mA
100mA
DC DISCONNECT
(I
MIN
)
LIMIT
(I
LIM
)
CUT
(I
CUT
)
200mA
300mA
400mA
500mA
CURRENT LIMIT
PORT OFF IN t
OVLD
DC DISCONNECT
PORT OFF IN t
MPDO
4263 F07
NORMAL
OPERATION
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