LTC4263-1
13
42631fa
APPLICATIONS INFORMATION
Thermal Protection
The LTC4263-1 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 LTC4263-1. At port turn-on, while
I
LIM
is active, the instantaneous power dissipated by the
LTC4263-1 can be as high as 18W. 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 LTC4263-1 into I
LIM
by drawing too much
current. Excessive heating can also occur if the V
DD5
pin
is shorted or overloaded.
The LTC4263-1 protects itself from thermal damage by
monitoring die temperature. If the die temperature exceeds
the overtemperature trip point, the LTC4263-1 removes
port power and shuts down all functions including the
internal 5V regulator. Once the die cools, the LTC4263-1
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
LTC4263-1 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 discon-
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
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 LTC4263-1 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 incre-
ments. 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 1k
resistor (R
DET
) from the DETECT pin to the port output as
shown in Figure 6. This provides an AC path for sensing
the port impedance. The 1k 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 being
shorted by the LTC4263-1’s power control MOSFET. The
500k resistor across D
AC
allows the port voltage to decay
after disconnect occurs.
Sizing of capacitors is critical to ensure proper function
of AC disconnect. C
PSE
(Figure 6) controls the connection
impedance on the PSE side. Its capacitance must be kept
low enough for AC disconnect to be able to sense the PD.
On the other hand, C
DET
has to be large enough to pass
the signal at 110Hz. The recommended values are 0.1μF
for C
PSE
and 0.47μF for C
DET
. The sizes of C
PSE
, C
DET
,
and R
DET
are chosen to create an economical, physically
Figure 5. Current Limit Foldback
V
DD48
– V
OUT
(V)
0
I
LIM
(mA)
450
600
750
40
42631 F07
300
150
0
51015
20 25
30 35 45
50
