LTC4266 LINER [Linear Technology], LTC4266 Datasheet - Page 23

• Manufacturer Part Number: LTC4266
• Description: Quad IEEE 802.3at Power over Ethernet Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Bypass capacitance between AGND and V
portant for reliable operation. If a short circuit occurs at
one of the output ports it can take as long as 1μs for the
LTC4266 to begin regulating the current. During this time
the current is limited only by the small impedances in the
circuit and a high current spike typically occurs, causing a
voltage transient on the V
the LTC4266 to reset due to a UVLO fault. A 1μF , 100V
X7R capacitor placed near the V
to minimize spurious resets.
Isolating the Serial Bus
The LTC4266 includes a split SDA pin (SDAIN and SDAOUT)
to ease opto-isolation of the bidirectional SDA line.
IEEE 802.3 Ethernet specifications require that network
segments (including PoE circuitry) be electrically isolated
from the chassis ground of each network interface device.
However, network segments are not required to be isolated
from each other, provided that the segments are connected
to devices residing within a single building on a single
power distribution system.
For simple devices such as small PoE switches, the isola-
tion requirement can be met by using an isolated main
power supply for the entire device. This strategy can be
used if the device has no electrically conducting ports
other than twisted-pair Ethernet. In this case, the SDAIN
and SDAOUT pins can be tied together and will act as a
standard I
V
0.22µF
EE
100V
C77
2
C/SMBus SDA pin.
10µF
C76
63V
10
10
R58
R60
+
EE
C78
0.22µF
100V
supply and possibly causing
2200pF
C79
R54
56k
EE
4.7k
R51
pin is recommended
1%
1
3
Figure 17. Positive V
ITH/RUN
V
4.7k
EE
R53
SUMIDA CDRH5D28-101NC
1%
FB
is very im-
LTC3803
GND
V
CC
5
2
100µH
L3
NGATE
SENSE
10µF
C75
16V
6
4
DD
R57
1k
If the device is part of a larger system, contains additional
external non-Ethernet ports, or must be referenced to
protective ground for some other reason, the Power over
Ethernet subsystem (including all LTC4266s) must be
electrically isolated from the rest of the system. Figure 18
shows a typical isolated serial interface. The SDAOUT pin
of the LTC4266 is designed to drive the inputs of an opto-
coupler directly. Standard I
cannot drive opto-couplers, so U1 is used to buffer the
signals from the host controller side.
External MOSFET
Careful selection of the power MOSFET is critical to system
reliability. LTC recommends either Fairchild IRFM120A,
FDT3612, FDMC3612 or Philips PHT6NQ10T for their
proven reliability in Type 1 and Type 2 PSE applications.
Non-standard applications that provide more current than
the 850mA IEEE maximum may require heat sinking and
other MOSFET design considerations. Contact LTC Ap-
plications before using a MOSFET other than one of these
recommended parts.
Sense Resistor
The LTC4266 is designed to use either 0.5Ω or 0.25Ω
current sense resistors. For new designs 0.25Ω is recom-
mended to reduce power dissipation; the 0.5Ω option is
intended for existing systems where the LTC4266 is used
as a drop-in replacement for the LTC4258 or LTC4259A.
The lower sense resistor values reduce heat dissipation.
Boost Converter
Q15
FDC2512
R59
0.100
1%, 1W
B1100
D28
100µF
6.3V
C74
FMMT723
4266 F17
Q13
SUMIDA CDRH4D28-100NC
R52
3.32k
1%
R55
806
1%
10µH
L4
R56
47.5k
1%
2
C/SMBus devices typically
Q14
FMMT723
10µF
6.3V
C73
LTC4266
3.3V AT 400mA
23
4266fb