Datasheet

LTC4274
25
4274fd
Sense Resistor
The LTC4274 is designed to use either 0.5 or 0.25
current sense resistors. For new designs 0.25 is recom-
mended to reduce power dissipation; the 0.5 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 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.
Output Cap
The port requires a 0.22F 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 V
EE
(Figure
10). An additional surge suppressor is required for each
LTC4274 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 LTC4274 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
Standard power layout guidelines apply to the LTC4274:
place the decoupling caps for the V
DD
and V
EE
supplies
near their respective supply pins, use ground planes, and
use wide traces wherever there are significant currents.
APPLICATIONS INFORMATION