LT4276
8
4276fa
For more information www.linear.com/LT4276
OVERVIEW
Power over Ethernet (PoE) continues to gain popularity
as products take advantage of DC power and high speed
data available from a single RJ45 connector. The LT4276A
allows higher power while maintaining backwards compat
-
ibility with existing PSE systems. The LT4276 combines
a PoE PD controller and a switching regulator controller
capable of either flyback or for
ward isolated power sup
-
ply operation.
SIGNIFICANT DIFFERENCES FROM PREVIOUS
PRODUCTS
The
LT4276 has several significant differences from pre
-
vious Linear Technology products. These differences are
briefly summarized below. See Applications Information
for more detail.
ITHB Is Inverted from the Usual ITH pin
The
ITHB pin voltage has an inverse relationship to the cur
-
rent sense comparator threshold, V
SENSE
. Furthermore, the
ITHB pin offset voltage, V
ITHB(OS)
, is 3.17V. See Figure 1.
Duty-Cycle Based Soft-Start
The LT4276 uses a duty cycle ramp soft-start that injects
charge into ITHB. This allows startup without appreciable
overshoot and with inexpensive external components.
The Feedback Pin (FB31) is 3.17V rather than 1.25V
The error amp feedback voltage (V
FB
) is 3.17V.
applicaTions inForMaTion
Figure 1. V
SENSE
vs. V
ITHB
Flyback/Forward Mode Is Pin Selectable
The LT4276 operates in flyback mode if FFSDLY is pulled
down by a resistor to GND. It operates in forward mode
if FFSDLY is pulled up by a resistor to V
CC
. The value of
this resistor determines the t
PGDELAY
and t
PGSG
.
T2P Pin Polarity Is Reversed
The T2P pin pulls up to V
CC
when active rather than pull-
ing down to GND.
V
CC
Is Powered by Internally Driven Buck Regulator
The LT4276 includes a buck regulator controller that must
be used to generate the V
CC
supply voltage.
PoE MODES OF OPERATION
The LT4276 has several modes of operation, depending
on the input voltage sequence applied to the VPORT pin.
V
SENSE
∆V
SENSE
∆V
ITHB
V
ITHB
V
ITHB(OS)
4276 F01
Table 1. Classification Codes, Power Levels and Resistor Selection
CLASS
PD POWER
AVAILABLE PD TYPE
NOMINAL CLASS
CURRENT
LT4276 GRADE CAPABILITY RESISTOR (1%)
A B C R
CLS
R
CLS
++
0 13W Type 1 0.7mA
√ √ √
Open Open
1 3.84W Type 1 10.5mA
√ √ √
150Ω Open
2 6.49W Type 1 18.5mA
√ √ √
80.6Ω Open
3 13W Type 1 28mA
√ √ √
52.3Ω Open
4 25.5W Type 2 40mA
√ √
35.7Ω Open
4* 38.7W LTPoE
++
40mA
√
Open 35.7Ω
4* 52.7W LTPoE
++
40mA
√
150Ω 47.5Ω
4* 70W LTPoE
++
40mA
√
80.6Ω 64.9Ω
4* 90W LTPoE
++
40mA
√
52.3Ω 118Ω
*An LTPoE
++
PD classifies as class 4 by an IEEE 802.3 compliant PSE.
