LTC4258 datasheet(19/28 Pages) LINER

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LTC4258

4258p

APPLICATIO S I FOR ATIO

face. Using transient suppressors, polyfuses and ex-

tended wait times after disconnecting a PD are effective

strategies to reduce the extremes applied to the external

MOSFETs.

Surge Suppressors and Circuit Protection

IEEE 802.3af Power over Ethernet is a challenging Hot Swap

application because it must survive the (probably unin-

tentional) abuse of everyone in the building. While hot

swapping boards in a networking or telecom card cage is

done by a trained technician or network administrator,

anyone in the building can plug a device into the network.

Moreover, in a card cage the physical domain being pow-

ered is confined to the card cage. With Power over Ether-

net, the PSE supplies power to devices up to 100 meters

away. Ethernet cables could potentially be cut, shorted

together, and so on by all kinds of events from a contrac-

tor cutting into walls to someone carelessly sticking a

screwdriver where it doesn’t belong. Consequently, the

Power over Ethernet power source (PSE) must be designed

to handle these events.

The most dramatic of these is shorting a powered port.

What the PSE sees depends on how much CAT-5 cable is

between it and the short. If the short occurs on the far end

of a long cable, the cable inductance will prevent the cur-

rent in the cable from increasing too quickly and the

LTC4258’s built-in short-circuit protection will take con-

trol of the situation and turn off the port. Some energy is

stored in the cable, but the transient suppressor on the

port clamps the port voltage when the cable inductance

causes the voltage to fly back after the MOSFET is turned

off. Because the cable only had 600mA or so going through

it, an SMAJ58A or equivalent device can easily control the

port voltage during flyback. With no cable connected at all,

a powered port shorted at the PSE’s RJ-45 connector can

reach high current levels before the port is shut down. There

is no cable inductance to store energy so once the port is

shut down the situation is under control.

A short—hence low inductance—piece of CAT-5 will not

limit the rapid increase of current when the port is shorted.

Even though the LTC4258 short-circuit shutdown is fast,

the cable may have many amps flowing through it before

the MOSFET can be turned off. Due to the high current,

this short piece of cable flies back with significant energy

behind it and must be controlled by the transient suppres-

sor. Choosing a surge suppressor that will not develop

more than a few volts of forward voltage while passing

more than 10A is important. A positive port voltage may

forward bias the detect diode (DDETn), bringing the

LTC4258’s DETECT

n pin positive as well and engaging

the DETECT

n clamps. This will generally not damage the

LTC4258 but extreme cases can cause the LTC4258 to

reset. When it resets, the LTC4258 signals an interrupt,

alerting the host controller which can then return the

LTC4258 to normal operating mode.

A substantial transient surge suppressor can typically

protect the LTC4258 and the rest of the PSE from these

faults. Placing a polyfuse between the RJ-45 connector

and the LTC4258 and its associated circuitry can provide

additional protection. To meet safety requirements, place

the polyfuse in the ground leg of the PSE’s output.

DC DISCONNECT

DC disconnect monitors the sense resistor voltage when-

ever the power is on to make sure that the PD is drawing

the minimum specified current. The disconnect timer

counts up whenever port current is below 7.5mA (typ). If

the tDIS timer runs out, the corresponding port will be

turned off and the disconnect bit in the fault register will be

set. If the undercurrent condition goes away before the

tDIS timer runs out, the timer will reset. The timer will start

counting from the beginning if the undercurrent condition

occurs again. The undercurrent circuit includes a glitch

filter to filter out noise.

The DC disconnect feature can be disabled by clearing the

corresponding DC Discon Enable bits in the Disconnect

by bits 1 and 0 of register 16h.

The LTC4258 implements a variety of current sense and

limit thresholds to control current flowing through the

port. Figure 14 is a graphical representation of these

thresholds and the action the LTC4258 takes when

currrent crosses the thresholds.

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