LTC4263CDE-1#TRPBF Datasheet LTC4263CDE-1#PBF, LTC4263IDE-1#PBF, LTC4263CDE-1#TRPBF | P16-P18

LTC4263-1

42631fa

APPLICATIONS INFORMATION

power if the port exceeds 540mA(min). This sets the

maximum system operating current and along with the

working voltage, limits the power available to the PD. The

power at the PSE output is reduced by the resistance of

the LTC4263-1 internal power MOSFET and the voltage

drop of the AC blocking diode if used for AC disconnect

as shown in Figure 8. The cabling can be responsible for

the largest loss. In our example based on a worst-case

100 meter CAT5 cable and connectors, the power loss can

be as much as 3.6W. Obviously for shorter cable runs the

loss is less and lower resistance cables such as CAT6 will

have correspondingly lower losses.

The total power available at the PD can be calculated taking

in to account these losses using the formula:

PV I I R

PD SU PPLY CUT CUT ON

DAC CUT

()

•– •

–•–

IIR

CUT CABLE

•

()

For many PoE systems, the only parameter affecting power

delivery that is under the control of the PSE designer is the

power supply. Optimum power delivery can be obtained

by maximizing this power supply voltage. 4-pair systems

can be treated as two independent 2-pair systems and

therefore the power will be twice that of the 2-pair.

2-Pair vs 4-Pair

One of the basic architectural decisions associated with

a high power PoE system is whether to deliver power

using four conductors (2-pair) or all eight conductors

(4-pair). Each method provides advantages and the

system vendor needs to decide which method best suits

their application.

2-pair power is used today in IEEE 802.3af systems (see

Figure 1). One pair of conductors is used to deliver the

current and a second pair is used for the return while two

conductor pairs are not powered. This architecture offers

the simplest implementation method but suffers from

higher cable loss than an equivalent 4-pair system.

4-pair power delivers current to the PD via two conductor

pairs in parallel (Figure 9). This lowers the cable resis-

tance but raises the issue of current balance between

each conductor pair. Differences in resistance of the

transformer, cable and connectors along with differences

in diode bridge forward voltage in the PD can cause an

imbalance in the currents ﬂ owing through each pair. The

4-pair system in Figure 9 solves this problem by using

two independent DC/DC converters in the PD. Using a

2-pair architecture with the LTC4263-1 allows delivery of

25W to the PD while using a 4-pair architecture allows

delivery of 50W. Contact Linear Technology applications

support for detailed information on implementing 2-pair

and 4-pair PoE systems.

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.

Figure 8. Example of Power Delivery Calculation Using the LTC4263-1

CUT(MIN)

= 540mA

ON(MAX)

= 3Ω

LTC4263-1

100M CAT5 CABLE

12.5Ω (MAX)

DISCONNECT

DIODE

(OPTIONAL)

42631 F08

POWER

SUPPLY

POWER (MIN)

= 55V (MIN) • 540mA (MIN)

= 29.7W (MIN)

LOSS_4263-1(MAX)

= (540mA)

• 3Ω

= 0.9W (MAX)

LOSS_AC DIODE(MAX)

= 0.5V • 540mA

= 0.3W (MAX)

OUT(MIN)

= 28.8W (MIN, DC DISCONNECT)

= 28.5W (MIN, AC DISCONNECT)

OUT(MIN)

= 25.2W (MIN, DC DISCONNECT)

= 24.9W (MIN, AC DISCONNECT)

CABLE LOSS (MAX)

= (540mA)

• 12.5Ω

= 3.6W

SUPPLY

55V TO 57V

LTC4263-1

42631fa

APPLICATIONS INFORMATION

Figure 9. 4-Pair High Power PoE Gigabit Ethernet System Diagram

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 intermittent

behavior to excessive voltages that may damage circuitry

in both the PSE and PD connected to the port.

Transient Suppressor Diode

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 LTC4263-1 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 ﬂ yback 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

LTC4263-1 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 LTC4263-1. An

SMAJ58A or equivalent device works well to maintain

port voltages within a safe range. A bidirectional transient

suppressor should not be used.

Good layout practices place the transient voltage sup-

pressor close to the LTC4263-1, before the common

mode choke (if used) and data magnetics to enhance the

protective function.

SMAJ58A

58V

0.1μF

–

56V

–

56V

DATA PAIR

PSE

0.1μF

OUT

DD48

DD5

LTC4263-1

SMAJ58A

58V

0.1μF

DATA PAIR

0.1μF

OUT

DD48

DD5

LTC4263-1

42631

SMAJ58A

8s B2100

CLASS

PWRGD

OUT

LTC4264

GND

DC/DC

CONVERTER

5μF

MIN

–

0.1μF

SMAJ58A

CLASS

PWRGD

OUT

LTC4264

GND

DC/DC

CONVERTER

5μF

MIN

0.1μF

LTC4263-1

42631fa

APPLICATIONS INFORMATION

If the port voltage reverses polarity and goes positive,

the OUT pin can be overstressed because this voltage is

stacked on top of the 56V supply. In this case, the transient

suppresser is used to clamp the voltage to a small positive

value to protect the LTC4263-1 and the PSE capacitor. For

this reason, it is critical that only a unidirectional TVS

be used.

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

sufﬁ cient margins should be used when selecting transient

suppressors for these applications.

Capacitors

Sizing of both the C

DET

and C

PSE

capacitors is critical for

proper operation of the LTC4263-1 AC disconnect sensing.

See the AC Disconnect section for more information. Note

that many ceramic capacitors have dramatic DC voltage

and temperature coefﬁ cients. Use 100V or higher rated

X7R capacitors for C

DET

and C

PSE

, 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 LTC4263-1. The V

DD5

supply

also requires a 0.1μF bypass capacitor.

Fuse

While the LTC4263-1 does not require a fuse for proper

operation, some safety requirements state that the output

current must be limited to less than 2A in less than 60

seconds if any one component fails or is shorted. Since

the LTC4263-1 is the primary current limiter, its failure

could result in excess current to the port. To meet these

safety requirements, a fuse can be placed in the positive

leg of the port. The fuse must be large enough that it will

pass at least 675mA when derated for high temperature

but small enough that it will fuse at less than 2A at cold

temperature. This requirement can usually be satisﬁ ed

with a 1A fuse or PTC. Placing the fuse between the RJ-45

connector and the LTC4263-1 and its associated circuitry

provides additional protection for this circuitry. Consult

a safety requirements expert for the application speciﬁ c

requirements.

Isolation

The IEEE 802.3af standard requires Ethernet ports to be

electrically isolated from all other conductors that are

user accessible. This includes the metal chassis, other

connectors, and the AC power line. Environment A isola-

tion is the most common and applies to wiring within a

single building serviced by a single AC power system.

For this type of application, the PSE isolation requirement

can be met with the use of a single, isolated 56V supply

powering several LTC4263-1 ports. Environment B, the

stricter isolation requirement, is for networks that cross

an AC power distribution boundary. In this case, electrical

isolation must be maintained between each port in the

PSE. The LTC4263-1 can be used to build a multi-port

Environment B PSE by powering each LTC4263-1 from

a separate, isolated 56V supply. In all PSE applications,

there should be no user accessible connections to the

LTC4263-1 other than the RJ-45 port.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

LTC4263CDE-1#TRPBF

Mfr. #:

Buy LTC4263CDE-1#TRPBF

Manufacturer:

Analog Devices Inc.

Description:

Power Switch ICs - POE / LAN IEEE 802.3af Single PSE Controller

Lifecycle:

New from this manufacturer.

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LTC4263CDE-1#TRPBF

LTC4263CDE-1#TRPBF

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