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LTC4274

4274fa

Sense Resistor

The LTC4274 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 LTC4274 is used

as a drop-in replacement for the LTC4258 or LTC4259A.

The lower sense resistor values reduce heat dissipation.

Four commonly available 1Ω resistors (0402 or larger

package size) can be used in parallel in place of a single

0.25Ω resistor. In order to meet the ICUT and ILIM accuracy

required by the IEEE specification, the sense resistors

should have ±1% tolerance or better, and no more than

±200ppm/°C temperature coefficient.

Output Cap

The port requires a 0.22μF cap across its output to keep

the LTC4274 stable while in current limit during startup

or overload. Common ceramic capacitors often have sig-

nificant voltage coefficients; this means the capacitance

is reduced as the applied voltage increases. To minimize

this problem, X7R ceramic capacitors rated for at least

100V are recommended.

ESD/Cable Discharge Protection

Ethernet ports can be subject to significant ESD events

when long data cables, each potentially charged to thou-

sands of volts, are plugged into the low impedance of the

RJ45 jack. To protect against damage, the port requires a

pair of clamp diodes; one to AGND and one to VEE (Figure

10). An additional surge suppressor is required for each

LTC4274 chip from VEE to AGND. The diodes at the port

steer harmful surges into the supply rails, where they are

absorbed by the surge suppressor and the VEE bypass

capacitance. The surge suppressor has the additional

benefit of protecting the LTC4274 from transients on the

VEE supply.

S1B diodes work well as port clamp diodes, and an

SMAJ58A or equivalent is recommended for the VEE surge

suppressor.

LAYOUT GUIDELINES

Standard power layout guidelines apply to the LTC4274:

place the decoupling caps for the VDD and VEE supplies

near their respective supply pins, use ground planes, and

use wide traces wherever there are significant currents.

APPLICATIONS INFORMATION

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