LTC4258 datasheet(25/28 Pages) LINER

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LTC4258

4258p

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

Ωsense resistor, RS,

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

VEE pin. Either of these strategies will prevent voltages

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 IINRUSH, ILIM, ICUT

and IMIN. Therefore, to maintain IEEE compliance, use a

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

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 DDET. Excessive

voltage drop across DDET will corrupt the LTC4258’s

detect and classification results. Select a diode for DDET

that will have less than 0.7V of forward drop at 0.4mA and

less than 0.9V of forward drop at 50mA.

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