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LTC4274

4274fa

APPLICATIONS INFORMATION

Figure 18. Opto-Isolating the I2C Bus

4274 F18

VDD

INT

SCL

SDAIN

SDAOUT

AD0

AD1

AD2

AD3

DGND

AGND

LTC4274

0.1 F

10 F

0.1 F

200

HCPL-063L

VDD CPU

SCL

SDA

SMBALERT

GND CPU

U1: FAIRCHILD NC7WZ17

U2, U3: AGILENT HCPL-063L

CONTROLLER

ISOLATED

3.3V

ISOLATED

GND

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 I2C/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 I2C/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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