LTC4268-1
14
42681fc
applicaTions inForMaTion
The PD must be able to handle power received in either
polarity. For this reason, it is common to install diode
bridges between the RJ45 connector and the LTC4268-1
(Figure 3). The diode bridges introduce an offset that
affects the threshold points for each range of operation.
The LTC4268-1 meets the IEEE 802.3af-defined operating
modes by compensating for the diode drops in the threshold
points. For the signature, classification, and the UVLO
thresholds, the LTC4268-1 extends two diode drops below
the IEEE 802.3af specifications. The LTC4268-1 threshold
points support the use of either traditional or Schottky
diode bridges.
Figure 2. V
IN
Voltage, PWRGD, PWRGD and PD Current
as a Function of Port Voltage
DETECTION V1
CLASSIFICATION
UVLO
TURN-ON
UVLO
OFF
POWER
BAD
UVLO
OFF
UVLO
ON
UVLO
TURN-OFF
?= R
LOAD
C1
PWRGD TRACKS
V
IN
DETECTION V2
10
TIME
20
30
40
50
10
TIME
20
30
V
40
50
10
TIME
20
30
40
50
20
10
I
CLASS
I
LIMIT_HIGH
I
LIMIT_LOW
dV
dt
I
LIMIT_LOW
C1
=
POWER
BAD
POWER
BAD
POWER
BAD
TIME
TIME
POWER
GOOD
POWER
GOOD
DETECTION I
1
CLASSIFICATION
DETECTION I
2
LOAD, I
LOAD
(UP TO I
LIMIT_HIGH
)
42681 F02
I
CLASS
DEPENDENT ON R
CLASS
SELECTION
I
LIMIT_LOW
= 300mA, I
LIMIT_HIGH
= 750mA
I
1
=
V1 2 DIODE DROPS
25k
I
LOAD
=
V
IN
R
LOAD
I
2
=
V2 2 DIODE DROPS
25k
V
PORTP
PSE
I
IN
R
LOAD
R
CLASS
V
IN
C1
R
CLASS
PWRGD
PWRGD
LTC4268-1
V
NEG
V
PORTN
V
PORT
+
RX
6
RX
+
3
TX
2
TX
+
RJ45
T1
POWERED
DEVICE (PD)
INTERFACE
AS DEFINED
BY IEEE 802.3af
42681 F03
1
7
8
5
4
SPARE
SPARE
+
TO PHY
BR2
0.1礔
100V
BR1
V
PORTP
D3
LTC4268-1
V
PORTN
Figure 3. PD Front End Using Diode Bridges on
Main and Spare Inputs
DETECTION
During detection, the PSE will apply a voltage in the range
of 2.8V to 10V on the cable and look for a 25k signature
resistor. This identifies the device at the end of the cable
as a PD. With the PSE voltage in the detection range, the
LTC4268-1 presents an internal 25k resistor between the
V
PORTP
and V
PORTN
pins. This precision, temperature-
compensated resistor provides the proper characteristics
to alert the PSE that a PD is present and requests power
to be applied.
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