Datasheet

LTC4274A/LTC4274C
27
4274acfc
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 LTC4274A/LTC4274C is designed to use either 0.5 or
0.25 current sense resistors. For new designs 0.25 is
recommended to reduce power dissipation; the 0.5 op-
tion is intended for existing systems where the LTC4274A/
LTC4274C is used as a drop-in replacement for the LTC4258
or LTC4259A. The lower sense resistor values reduce
heat dissipation. Four commonly available 1 resistors
(0402 or larger package size) can be used in parallel in
place of a single 0.25 resistor. In order to meet the I
CUT
and I
LIM
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
The port requires a 0.22F cap across its output to keep
the LTC4274A/LTC4274C 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, the port requires a pair
of clamp diodes; one to AGND and one to V
EE
(Figure 10).
An additional surge suppressor is required for each
LTC4274A/LTC4274C chip from V
EE
to AGND. The diodes
at the port steer harmful surges into the supply rails,
where they are absorbed by the surge suppressor and the
V
EE
bypass capacitance. The surge suppressor has the
additional benefit of protecting the LTC4274A/LTC4274C
from transients on the V
EE
supply.
S1B diodes work well as port clamp diodes, and an
SMAJ58A or equivalent is recommended for the V
EE
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 com-
ponent 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