LTC4271 datasheet(27/30 Pages) LINER

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LTC4266A/LTC4266C

4266acfc

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 LTC4266A/LTC4266C is designed to use either 0.5Ω or

0.25Ω current sense resistors. For new designs 0.25Ω is

recommended to reduce power dissipation; the 0.5Ω op-

tion is intended for existing systems where the LTC4266A/

LTC4266C 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.

Port Output Cap

Each port requires a 0.22μF cap across its outputs to

keep the LTC4266A/LTC4266C stable while in current limit

during startup or overload. Common ceramic capacitors

often have significant 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 thousands

of volts, are plugged into the low impedance of the RJ45

jack. To protect against damage, each port requires a pair

of clamp diodes; one to AGND and one to VEE (Figure 10).

An additional surge suppressor is required for each

LTC4266A/LTC4266C chip from VEE to AGND. The diodes

at the ports 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 LTC4266A/LTC4266C

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

Strict adherence to board layout, parts placement and

routing guidelines is critical for optimal current read-

ing accuracy, IEEE compliance, system robustness, and

thermal dissipation.

Power delivery above 25.5W imposes additional compo-

nent and layout restraints. Specifically MOSFET, sense

resistor and transformer selection is crucial to safe and

reliable system operation.

Contact LTC Applications to obtain a full set of layout

guidelines, example layouts and BOMs.

APPLICATIONS INFORMATION

LTC4271 Datasheet(PDF) 27 Page - Linear Technology

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