ltc4263cde-trpbf Linear Technology Corporation, ltc4263cde-trpbf Datasheet - Page 14

• Manufacturer Part Number: ltc4263cde-trpbf
• Description: Single Ieee 802.3af Compliant Pse Controller With Internal Switch
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC4263
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 satisfi es
the PD’s power needs while minimizing disturbances on
the 48V backplane.
Port Overload
A PSE port is permitted to supply up to 15.4W continuously
and up to 400mA (I
overload. Per the IEEE 802.3af specifi 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
14
500mA
400mA
300mA
200mA
100mA
0mA
Figure 6. Current Thresholds and Current Limits
DC DISCONNECT
INRUSH
(I
MIN
)
CUT
and I
) for up to 75ms (t
(I
CUT
CUT
LIM
)
, that a PSE must implement.
LIMIT
(I
LIM
)
NORMAL
OPERATION
CURRENT LIMIT
PORT OFF IN t
DC DISCONNECT
PORT OFF IN t
OVLD
) when in
OVLD
MPDO
4263 F07
I
mum allowable current in the case of a short after the port is
powered. Because the IEEE specifi cation calls out identical
values, the LTC4263 implements both as a single current
limit referred to as I
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
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
650mA (typ)
such an event occurs. The circuit then limits current to I
while the t
I
t
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
timer to monitor overload conditions. The timer is incre-
mented whenever current greater than I
the port. If the current is still above I
timer expires, the LTC4263 will turn off power to the port
and fl ash the LED. In this situation, the LTC4263 waits
four seconds and then restarts detection. If the overload
INRUSH
LIM
OVLD
will be reduced by the foldback circuitry.
Timing
refers to current at port turn-on and I
OVLD
)
and quickly turns off the internal MOSFET if
timer increments. During a short-circuit,
LIM
.
CUT
CUT
when the t
LIM
fl ows through
LIM
is the maxi-
for port
(
I
OUT
OVLD
OVLD
4263fc
LIM
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