ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 15

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
OPERATION
Note: Please refer to the simplifi ed application circuit
(Figure 2) for voltage naming conventions used in this
datasheet.
The LTC4268-1 high power PD interface controller and
switching regulator has several modes of operation depend-
ing on the applied V
summarized in Table 1. These various modes satisfy the
requirements defi ned in the IEEE 802.3af specifi cation. The
input voltage is applied to the V
to the V
SERIES DIODES
The IEEE 802.3af-defi ned operating modes for a PD refer-
ence the input voltage at the RJ45 connector on the PD.
In this datasheet port voltage is normally referenced to
the pins of the LTC4268-1. Note that the voltage ranges
specifi ed in the LTC4268-1 Electrical Specifi cations are
referenced with respect to the IC pins.
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 4). The diode bridges introduce an offset that
affects the threshold points for each range of operation.
The LTC4268-1 meets the IEEE 802.3af-defi ned operating
modes by compensating for the diode drops in the threshold
points. For the signature, classifi cation, and the UVLO
PORTP
RJ45
1
2
3
6
4
5
7
8
TX
TX
RX
RX
pin and is always negative.
+
–
+
–
SPARE
SPARE
PORT
+
–
16
15
14
11
10
voltage as shown in Figure 3 and
9
Figure 2. Simplifi ed Application Circuit with Voltage Naming Conventions
T1
1
2
3
6
7
8
PORTN
PD FRONT END
TO PHY
pin with reference
~
~
~
~
+
–
+
–
V
PORT
thresholds, the LTC4268-1 extends two diode drops below
the IEEE 802.3af specifi cations. The LTC4268-1 threshold
points support the use of either traditional or Schottky
diode bridges.
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 identifi es 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
compensated resistor provides the proper characteristics
to alert the PSE that a PD is present and requests power
to be applied.
Table 1. LTC4268-1 Operational Mode as a Function
of V
0V to –1.4V
–1.5V to –10.1V
–10.3V to –12.4V
–12.5V to UVLO*
UVLO* to –57V
*UVLO includes hysteresis.
Rising input threshold ≅ –38.9V
Falling input threshold ≅ –30.6V
PORTP
V
PORTN
PORT
LTC4268-1
V
PORTP
V
PORT
Voltage
SWITCHING REGULATOR
and V
V
GND
V
NEG
PG
CC
PORTN
V
IN
+
Inactive
25k Signature Resistor Detection
Classifi cation Load Current Ramps Up from 0%
to 100%
Classifi cation Load Current Active
Power Applied to PD Load
pins. This precision, temperature-
•
MODE OF OPERATION
•
ISOLATED OUTPUT
LTC4268-1
+
V
42681 F02
OUT
15
42681fa