LTC4263CS datasheet(18/24 Pages) LINER

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LTC4263

4263fe

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 reduction 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

occurs 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

suppressor 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

application. There is a potential for stress induced leakage,

so sufﬁcient margins should be used when selecting

transient suppressors for these applications.

Capacitors

Sizing of both the CDET and CPSE capacitors is critical for

proper operation of the LTC4263 AC disconnect sensing.

See the AC Disconnect section for more information. Note

that many ceramic capacitors have dramatic DC voltage

and temperature coefﬁcients. Use 100V or higher rated

X7R capacitors for CDET and CPSE, as these have reduced

voltage dependence while also being relatively small and

inexpensive. Bypass the 48V supply with a 0.1μF, 100V

capacitor located close to the LTC4263. The VDD5 supply

also requires a 0.1μF bypass capacitor.

LTC4263CS Datasheet(PDF) 18 Page - Linear Technology

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