LTC4266 LINER [Linear Technology], LTC4266 Datasheet - Page 26

• Manufacturer Part Number: LTC4266
• Description: Quad IEEE 802.3at Power over Ethernet Controller
• Manufacturer: LINER [Linear Technology]

LTC4266
APPLICATIONS INFORMATION
a perfect Kelvin connection because all four ports con-
trolled by the LTC4266 share the same sense trace, and
because the current through the trace (I
However, as the equation shows, the remaining error is a
small offset term.
Figure 20 shows two LTC4266 chips controlling eight ports
(A though H). The ports are separated into two groups
of four; each has its own trace on the top PCB layer that
connects to the V
sub-circuit are effectively isolated from the U2 sub-circuit,
reducing the layout problem down to 4-port chunks; this
arrangement can be expanded for any number of ports.
Figure 21 shows an example of good 4-port layout. Each
0.25Ω sense resistor consists of four 1Ω resistors in
26
THIS TRACE PROVIDES V
BUT ALSO ACTS AS A KELVIN
SENSE LINE FOR PORTS A-D
EE
plane with a via. Currents from the U1
U1
LTC4266
EE
SENSE1
SENSE2
SENSE3
SENSE4
TO U1
V
EE
V
V
EE
Figure 20. Layout Strategy to Reduce Mutual Resistance
EE
COPPER FILL ON SURFACE LAYER
PLANE ON INNER LAYER
PORTS A THROUGH D
EE
) is not zero.
VIA
parallel. The four groups of resistors are arranged to
minimize the overlap in their current flows, which mini-
mizes mutual resistance. The horizontal slits cut in the
copper help to keep the currents separate. Wide copper
paths connect each group of resistors to the vias at the
center, so the resistance is very low.
Proper connection of the sense line is also important. In
Figure 21, U1 is not connected directly to the V
but is connected instead to a Kelvin sense trace that
leads to the sense resistor array. Similarly, the via at the
center of the sense resistor array has a matching hole
in the V
resistance of the four large vias from influencing the
current measurements.
BY KEEPING THESE COPPER FILLS SEPARATE ON
THE SURFACE, MUTUAL RESISTANCE BETWEEN
PORTS A-D AND E-H IS ELIMINATED
U2
LTC4266
SENSE1
SENSE2
SENSE3
SENSE4
V
EE
EE
plane. This arrangement prevents the mutual
PORTS E THROUGH H
VIA
R
SENSE
RETURN TO
V
EE
POWER SUPPLY
4266 F20
EE
plane
4266fb