LTC4263 datasheet(18/24 Pages) LINER

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LTC4263

4263fd

APPLICATIONS INFORMATION

Common Mode Chokes

Both non-powered and powered Ethernet connections

achieve best performance for data transfer and EMI when a

common mode choke is used on each port. For cost reduc-

tion reasons, some designs share a common mode choke

between two adjacent ports. This is not recommended.

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 IEEE 802.3af to intermittent behavior

and even to excessive voltages that may damage circuitry

in both the PSE and PD connected to the port.

Transient Suppressor Diode

IEEE 802.3af Power over Ethernet is a challenging Hot

Swap™ application because it must survive unintentional

abuse by repeated plugging in and out of devices at the

port. Ethernet cables could potentially be cut or shorted

together. Consequently, the PSE must be designed to

handle these events without damage.

The most severe of these events is a sudden short on a

powered port. What the PSE sees depends on how much

CAT-5 cable is between it and the short. If the short oc-

curs on the far end of a long cable, the cable inductance

will prevent the current in the cable from increasing too

quickly and the LTC4263 built-in short-circuit protection

will control the current and turn off the port. However,

the high current along with the cable inductance causes

a large ﬂyback voltage to appear across the port when the

MOSFET is turned off. In the case of a short occurring

with a minimum length cable, the instantaneous current

can be extremely high due to the lower inductance. The

LTC4263 has a high speed fault current limit circuit that

shuts down the port in 20μs (typ). In this case, there is

lower inductance but higher current so the event is still

severe. A transient suppressor is required to clamp the port

voltage and prevent damage to the LTC4263. An SMAJ58A

or equivalent device works well to maintain port voltages

within a safe range. A bidirectional transient suppressor

should not be used. Good board layout places the transient

suppressor between the port and the LTC4263 to enhance

the protective function.

If the port voltage reverses polarity and goes positive, the

OUT pin can be overstressed because this voltage is stacked

on top of the 48V supply. In this case, the transient sup-

pressor must clamp the voltage to a small positive value

to protect the LTC4263 and the PSE capacitor.

Component leakages across the port can have an adverse

affect on AC disconnect and even affect DC disconnect if

the leakage becomes severe. The SMAJ58A is rated at less

than 5μA leakage at 58V and works well in this applica-

tion. There is a potential for stress induced leakage, so

sufﬁcient margins should be used when selecting transient

suppressors for these applications.

Hot Swap is a trademark of Linear Technology Corporation.

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