LTC4263
15
4263fe
APPLICATIONS INFORMATION
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 during power-up and momentary short-circuit
conditions. At low port output voltages, the voltage
across the internal MOSFET is high, and power dissipa-
tion will be large if signifi cant current is fl owing. 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 7.
Figure 7. Current Limit Foldback
Thermal Protection
The LTC4263 includes thermal overload protection in
order to provide full device functionality in a miniature
package while maintaining safe operating temperatures.
Several factors create the possibility for very large power
dissipation within the LTC4263. At port turn-on, while
I
LIM
is active, the instantaneous power dissipated by the
LTC4263 can be as high as 12W. This can cause 40ºC or
more of die heating in a single turn-on sequence. Similarly,
excessive heating can occur if an attached PD repeatedly
pushes the LTC4263 into I
LIM
by drawing too much cur-
rent. Excessive heating can also occur if the V
DD5
pin is
shorted or overloaded.
The LTC4263 protects itself from thermal damage by
monitoring die temperature. If the die temperature exceeds
the overtemperature trip point, the LTC4263 removes port
power and shuts down all functions including the internal
5V regulator. Once the die cools, the LTC4263 waits four
seconds, then restarts detection.
DC Disconnect
The DC disconnect circuit monitors port current whenever
power is on to detect continued presence of the PD. IEEE
802.3af mandates a minimum current of 10mA that the
PD must draw for periods of at least 75ms with optional
dropouts of no more than 250ms. The t
MPDO
disconnect
timer increments whenever port current is below 7.5mA
(typ). If the timer expires, the port is turned off and the
LTC4263 waits 1.5 seconds before restarting detection. If
the undercurrent condition goes away before t
MPDO
(350ms
(typ)
), the timer is reset to zero. The DC disconnect circuit
includes a glitch fi lter to prevent noise from falsely resetting
the timer. The current must be present for a period of at
least 20ms to guarantee reset of the timer. To enable DC
disconnect, tie the OSC pin to V
SS
.
AC Disconnect
AC disconnect is an alternate method of sensing the pres-
ence or absence of a PD by monitoring the port impedance.
The LTC4263 forces an AC signal from an internal sine wave
generator on to the port. The ACOUT pin current is then
sampled once per cycle and compared to I
ACDMIN
. Like DC
disconnect, the AC disconnect sensing circuitry controls the
t
MPDO
disconnect timer. When the connection impedance
rises due to the removal of the PD, AC peak current falls
below I
ACDMIN
and the disconnect timer increments. If the
impedance remains high (AC peak current remains below
I
ACDMIN
), the disconnect timer counts to t
MPDO
and the
port is turned off. If the impedance falls, causing AC peak
current to rise above I
ACDMIN
for two consecutive samples
before the maximum count of the disconnect timer, the
timer resets and the port remains powered.
The AC disconnect circuitry senses the port via the ACOUT
pin. Connect a 0.47μF 100V X7R capacitor (C
DET
) and
a 1kΩ resistor (R
DET
) from the DETECT pin to the port
output as shown in Figure 8. This provides an AC path for
sensing the port impedance. The 1kΩ resistor, R
DET
, limits
current fl owing through this path during port power-on and
power-off. An AC blocking diode (D
AC
) is inserted between
the OUT pin and the port to prevent the AC signal from
V
DD48
– V
OUT
(V)
0
I
LIM
(mA)
300
400
500
40
4263 F07
200
100
0
51015
20 25
30 35 45
50
