LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 25

• Manufacturer Part Number: LTC4258
• Description: Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
a PSE to be noncompliant. For further assistance please
contact Linear Technology’s Applications department.
Sense Resistors
The LTC4258 is designed to use a 0.5 sense resistor, R
to monitor the current through each port. The value of the
sense resistor has been minimized in order to reduce
power loss and as a consequence, the voltage which the
LTC4258 must measure is small. Each port may be draw-
ing up to 450mA with this current flowing through the
sense resistor and associated circuit board traces. To
prevent parasitic resistance on the circuit board from
obscuring the voltage drop across the sense resistor, the
LTC4258 must Kelvin sense the resistor voltage. One way
to achieve Kelvin sensing is “star grounding,” shown
pictorially in Figure 1. Another option is to use a – 48V
power plane to connect the sense resistor and the LTC4258
V
developed across parasitic circuit board resistances from
affecting the LTC4258 current measurement accuracy.
The precision of the sense resistor directly affects the
measurement of the IEEE parameters I
and I
resistor with 0.5% or better accuracy.
Power MOSFETs
The LTC4258 controls power MOSFETs in order to regu-
late current flow through the Ethernet ports. Under certain
conditions these MOSFETs have to dissipate significant
power. See the Choosing External MOSFETs section for a
detailed discussion of the requirements these devices
must meet.
Common Mode Chokes
Both nonpowered and powered Ethernet connections
achieve best performance (for data transfer, power trans-
fer and EMI) when a common mode choke is used on each
port. In the name of cost reduction, some designs share a
EE
pin. Either of these strategies will prevent voltages
MIN
. Therefore, to maintain IEEE compliance, use a
U
U
W
INRUSH
, I
U
LIM
, I
CUT
S
,
common mode choke between two adjacent ports. Even
for nonpowered Ethernet, sharing a choke is not recom-
mended. With two ports passing through the choke, it
cannot limit the common mode current of either port.
Instead, the choke only controls the sum of both ports’
common mode current. Because cabling from the ports
generally connects to different devices up to 200m apart,
a current loop can form. In such a loop, common mode
current flows in one port and out the other, and the choke
will not prevent this because the sum of the currents is
zero. Another way to view this interaction between the
paired ports is that the choke acts as a transformer
coupling the ports’ common modes together. In
nonpowered Ethernet, common mode current results
from nonidealities like ground loops; it is not part of
normal operation. However, Power over Ethernet sends
power and hence significant current through the ports;
common mode current is a byproduct of normal opera-
tion. As described in the Choosing External MOSFETs
section and under the Power Supplies heading below,
large transients can occur when a port’s power is turned
on or off. When a powered port is shorted (see Surge
Suppressors and Circuit Protection), a port’s common
mode current may be excessive. Sharing a common mode
choke between two ports couples start-up, disconnect
and fault transients from one port to the other. The end
result can range from momentary noncompliance with
802.3af to intermittent behavior and even to excessive
voltages that may damage circuitry (in both the PSE and
PD) connected to the ports.
Detect, AC Blocking and Transient Supressor Diodes
During detection and classification, the LTC4258 senses
the port voltage through the detect diodes D
voltage drop across D
detect and classification results. Select a diode for D
that will have less than 0.7V of forward drop at 0.4mA and
less than 0.9V of forward drop at 50mA.
DET
will corrupt the LTC4258’s
LTC4258
DET
. Excessive
25
4258p
DET