Datasheet

LTC4266
23
4266fe
For more information www.linear.com/LTC4266
ApplicAtions inForMAtion
Bypass capacitance between AGND and V
EE
is very im-
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
EE
supply and possibly causing
the LTC4266 to reset due to a UVLO fault. A 1μF, 100V
X7R capacitor placed near the V
EE
pin is recommended
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
2
C/SMBus SDA pin.
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
2
C/SMBus devices typically
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.
Figure 17. Positive V
DD
Boost Converter
4266 F17
R54
56k
C79
2200pF
GND
ITH/RUN
LTC3803
V
CC
2
5
V
FB
1
3
NGATE
Q15
FDC2512
Q13
FMMT723
Q14
FMMT723
SENSE
6
4
V
EE
C74
100µF
6.3V
C75
10µF
16V
L3
100µH
SUMIDA CDRH5D28-101NC
R51
4.7k
1%
R53
4.7k
1%
R52
3.32k
1%
3.3V AT 400mA
R55
806Ω
1%
R59
0.100Ω
1%, 1W
R56
47.5k
1%
R57
1k
D28
B1100
R58
10Ω
R60
10Ω
C73
10µF
6.3V
L4
10µH
SUMIDA CDRH4D28-100NC
+
C77
0.22µF
100V
C78
0.22µF
100V
C76
10µF
63V