LTC4263 LINER [Linear Technology], LTC4263 Datasheet - Page 15

• Manufacturer Part Number: LTC4263
• Description: Single IEEE 802.3af Compliant PSE Controller with Internal Switch
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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
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
rent. Excessive heating can also occur if the V
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
condition is removed before the t
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
current limit level for port voltages of less than 28V, as
shown in Figure 7.
LIM
is active, the instantaneous power dissipated by the
100
500
400
300
200
0
0
Figure 7. Current Limit Foldback
5
10 15 20 25 30 35
V
DD48
LIM
– V
OUT
by drawing too much cur-
(V)
OVLD
40
timer expires, the
45 50
4263 F07
DD5
pin is
LIM
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
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
(
nect 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
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
disconnect, the AC disconnect sensing circuitry controls the
t
rises due to the removal of the PD, AC peak current falls
below I
impedance remains high (AC peak current remains below
I
port is turned off. If the impedance falls, causing AC peak
current to rise above I
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
a 1kΩ resistor (R
output as shown in Figure 8. This provides an AC path for
sensing the port impedance. The 1kΩ resistor, R
current fl owing through this path during port power-on and
power-off. An AC blocking diode (D
the OUT pin and the port to prevent the AC signal from
MPDO
ACDMIN
350ms (typ)
disconnect timer. When the connection impedance
ACDMIN
), the disconnect timer counts to t
)
, the timer is reset to zero. The DC discon-
and the disconnect timer increments. If the
DET
ACDMIN
) from the DETECT pin to the port
for two consecutive samples
AC
) is inserted between
LTC4263
MPDO
ACDMIN
MPDO
disconnect
SS
DET
DET
. Like DC
.
and the
15
, limits
) and
MPDO
4263fd