LTC4263 datasheet(18/42 Pages) LINER

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LTC4278

4278fc

PWRGD PIN WHEN SHDN IS INVOKED

InPDapplicationswhereanauxiliarypowersupplyinvokes

the SHDN feature, the PWRGD pin becomes high imped-

ance. This prevents the PWRGD pin that is connected to

the UVLO pin from interfering with the DC/DC converter

operations when powered by an auxiliary power supply.

OVERVOLTAGE LOCKOUT

The LTC4278 includes an overvoltage lockout (OVLO)

feature (Figure 6) which protects the LTC4278 and its load

from an overvoltage event. If the input voltage exceeds the

OVLO threshold, the LTC4278 discontinues PD operation.

Normal operations resume when the input voltage falls

below the OVLO threshold and when C1 is charged up.

THERMAL PROTECTION

TheIEEE802.3af/atspecificationrequiresaPDtowithstand

any applied voltage from 0V to 57V indefinitely. However,

there are several possible scenarios where a PD may

encounter excessive heating.

During classification, excessive heating may occur if the

PSEexceedsthe75msprobingtimelimit.Atturn-on,when

the load capacitor begins to charge, the instantaneous

power dissipated by the PD interface can be large before

it reaches the line voltage. And if the PD experiences a

fast input positive voltage step in its operational mode

(for example, from 37V to 57V), the instantaneous power

dissipated by the PD Interface can be large.

The LTC4278 includes a thermal protection feature which

protects the LTC4278 from excessive heating. If the

LTC4278 junction temperature exceeds the over-temper-

ature threshold, the LTC4278 discontinues PD operations

and power good becomes inactive. Normal operation

resumes when the junction temperature falls below the

overtemperature threshold and when C1 is charged up.

EXTERNAL INTERFACE AND COMPONENT SELECTION

Transformer

Nodes on an Ethernet network commonly interface to the

outside world via an isolation transformer. For PDs, the

isolation transformer must also include a center tap on

the RJ45 connector side (see Figure 7).

The increased current levels in a Type 2 PD over a Type

1 increase the current imbalance in the magnetics which

can interfere with data transmission. In addition, proper

termination is also required around the transformer to

provide correct impedance matching and to avoid radiated

and conducted emissions. Transformer vendors such as

Bel Fuse, Coilcraft, Halo, Pulse, and Tyco (Table 4) can

assist in selecting an appropriate isolation transformer

and proper termination methods.

Table 4. Power over Ethernet Transformer Vendors

VENDOR

CONTACT INFORMATION

Bel Fuse Inc.

206 Van Vorst Street

Jersey City, NJ 07302

Tel: 201-432-0463

www.belfuse.com

Coilcraft Inc.

1102 Silver Lake Road

Gary, IL 60013

Tel: 847-639-6400

www.coilcraft.com

Halo Electronics

1861 Landings Drive

Mountain View, CA 94043

Tel: 650-903-3800

www.haloelectronics.com

PCA Electronics

16799 Schoenborn Street

North Hills, CA 91343

Tel: 818-892-0761

www.pca.com

Pulse Engineering

12220 World Trade Drive

San Diego, CA 92128

Tel: 858-674-8100

www.pulseeng.com

Tyco Electronics

308 Constitution Drive

Menlo Park, CA 94025-1164

Tel: 800-227-7040

www.circuitprotection.com

Input Diode Bridge

Figure 2 shows how two diode bridges are typically con-

nected in a PD application. One bridge is dedicated to the

data pair while the other bridge is dedicated to the spare

pair. The LTC4278 supports the use of either silicon or

Schottky input diode bridges. However, there are tradeoffs

in the choice of diode bridges.

APPLICATIONS INFORMATION

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