LTC4263-1_15 Datasheet, PDF (12/20 Pages) Linear Technology – High Power Single PSE Controller with Internal Switch

LTC4263-1
APPLICATIONS INFORMATION
Power Control
The primary function of the LTC4263-1 is to control the
delivery of power to the PSE port. In order to provide
a robust solution, a variety of current limit and current
monitoring functions are needed, as shown in Figure 4. All
control circuitry is integrated and the LTC4263-1 requires
no external MOSFET, sense resistor, or microcontroller.
The LTC4263-1 includes an internal MOSFET for driving
the PSE port. The LTC4263-1 drives the gate of the internal
MOSFET while monitoring the current and the output volt-
age at the OUT pin. This circuitry couples the 56V input
supply to the port in a controlled manner that satisfies
the PD’s power needs while minimizing disturbances on
the 56V backplane.
750mA
600mA
CURRENT LIMIT
PORT OFF IN t_OVLD
450mA
300mA
NORMAL
OPERATION
150mA
0mA
DC DISCONNECT CUT
(I_MIN)
(I_CUT)
LIMIT
(I_LIM)
DC DISCONNECT
PORT OFF IN t_MPDO
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Figure 4. Current Thresholds and Current Limits
Port Overload
Based on the IEEE 802.3af standard, the LTC4263-1 detects
port overload conditions by monitoring port current. This
ensures the port stays within the designed continuous
power budget while allowing for brief power surges. If
the port current exceeds 570mA (typ) for greater than
62ms (typ), power is removed and the LTC4263-1 waits
4 seconds (typ) before returning to detection mode.
Port Inrush and Short-Circuit
When 56V power is applied to the port, the LTC4263-1 is
designed to power-up the PD in a controlled manner with-
out causing transients on the input supply. To accomplish
this, the LTC4263-1 implements inrush current limit. At
turn-on, current limit will allow the port voltage to quickly
12
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 LTC4263-1 limits port current to avoid a
hazardous 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 >
1000mA (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
LTC4263-1 includes a 62ms (typ) t_OVLD timer to limit the
duration of these events. The timer is incremented when-
ever current greater than I_CUT flows through the port. If
the current is still above I_CUT when the t_OVLD timer expires,
the LTC4263-1 will turn off power to the port and flash the
LED. In this situation, the LTC4263-1 waits four seconds
and then restarts detection. If the overload condition is
removed before the t_OVLD timer expires, the port stays
powered and the timer is reset.
Foldback
Foldback is designed to limit power dissipation in the
LTC4263-1 during power-up and momentary short-cir-
cuit conditions. At low port output voltages, the voltage
across the internal MOSFET is high, and power dissipa-
tion will be large if significant current is flowing. Foldback
monitors the port output voltage and reduces the I_LIM
current limit level for port voltages of less than 28V, as
shown in Figure 5.
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