LTC4271 datasheet(25/30 Pages) LINER

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LTC4266A/LTC4266C

4266acfc

APPLICATIONS INFORMATION

VEE is the main supply that provides power to the PDs.

Because it supplies a relatively large amount of power and

is subject to significant current transients, it requires more

design care than a simple logic supply. For minimum IR

loss and best system efficiency, set VEE near maximum

amplitude (57V), leaving enough margin to account for

transient over- or undershoot, temperature drift, and the

line regulation specs of the particular power supply used.

Bypass capacitance between AGND and VEE 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 LTC4266A/LTC4266C 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 VEE supply and

possibly causing the LTC4266A/LTC4266C to reset due to

a UVLO fault. A 1μF, 100V X7R capacitor placed near the

VEE pin is recommended to minimize spurious resets.

Isolating the Serial Bus

The LTC4266A/LTC4266C 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 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 LTC4266A/LTC4266Cs)

must be electrically isolated from the rest of the system.

Figure 16 shows a typical isolated serial interface. The

SDAOUT pin of the LTC4266A/LTC4266C 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.

Figure 15. Positive VDD Boost Converter

4266AC F15

R54

56k

C79

2200pF

GND

ITH/RUN

LTC3803

VCC

VFB

NGATE

Q15

FDC2512

Q13

FMMT723

Q14

FMMT723

SENSE

VEE

C74

100μF

6.3V

C75

10μF

16V

100μH

SUMIDA CDRH5D28-101NC

R51

4.7k

R53

4.7k

R52

3.32k

3.3V AT 400mA

R55

806Ω

R59

0.100Ω

1%, 1W

R56

47.5k

R57

D28

B1100

R58

10Ω

R60

10Ω

C73

10μF

6.3V

10μH

SUMIDA CDRH4D28-100NC

C77

0.22μF

100V

C78

0.22μF

100V

C76

10μF

63V

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