LTC4274
24
4274fd
APPLICATIONS INFORMATION
Figure 18. Opto-Isolating the I
2
C Bus
4274 F18
V
DD
INT
SCL
SDAIN
SDAOUT
AD0
AD1
AD2
AD3
DGND
AGND
LTC4274
0.1F
10F
2k
2k
0.1F
0.1F
200
200
200
200
U2
U3
U1
HCPL-063L
HCPL-063L
V
DD
CPU
SCL
SDA
SMBALERT
GND CPU
U1: FAIRCHILD NC7WZ17
U2, U3: AGILENT HCPL-063L
TO
CONTROLLER
ISOLATED
3.3V
ISOLATED
GND
+
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 LTC4274s) must be
electrically isolated from the rest of the system. Figure 18
shows a typical isolated serial interface. The SDAOUT pin
of the LTC4274 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.
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