LTC4263 datasheet(14/24 Pages) LINER

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LTC4263

4263fd

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 satisﬁes

the PD’s 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 (ICUT) for up to 75ms (tOVLD) when in

overload. Per the IEEE 802.3af speciﬁcation, 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, IINRUSHandILIM,thataPSEmustimplement.

IINRUSHreferstocurrentatportturn-onandILIMisthemaxi-

mum allowable current in the case of a short after the port is

powered. Because the IEEE speciﬁcation calls out identical

values, the LTC4263 implements both as a single current

limit referred to as ILIM.

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 ILIM 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

(IOUT >

650mA (typ)

) and quickly turns off the internal MOSFET if

such an event occurs. The circuit then limits current to ILIM

while the tOVLD timer increments. During a short-circuit,

ILIM will be reduced by the foldback circuitry.

tOVLD 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) tOVLD

timer to monitor overload conditions. The timer is incre-

mented whenever current greater than ICUT ﬂows through

the port. If the current is still above ICUT when the tOVLD

timer expires, the LTC4263 will turn off power to the port

and ﬂash the LED. In this situation, the LTC4263 waits

four seconds and then restarts detection. If the overload

0mA

100mA

DC DISCONNECT

(IMIN)

LIMIT

(ILIM)

CUT

(ICUT)

200mA

300mA

400mA

500mA

CURRENT LIMIT

PORT OFF IN tOVLD

DC DISCONNECT

PORT OFF IN tMPDO

4263 F07

NORMAL

OPERATION

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