LTC4267CGN-1#TRPBF Datasheet LTC4267IGN-1#PBF, LTC4267CGN-1#PBF, LTC4267CGN-1#TRPBF | P16-P18

LTC4267-1

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The undervoltage lockout mechanism on P

VCC

prevents

the LTC4267-1 switching regulator from trying to drive

the external N-Channel MOSFET with insufficient gate-to-

source voltage. The voltage at the P

VCC

pin must exceed

TURNON

(nominally 8.7V with respect to PGND) at least

momentarily to enable operation. The P

VCC

voltage must

fall to V

TURNOFF

(nominally 5.7V with respect to PGND)

before the undervoltage lockout disables the switching

regulator. This wide UVLO hysteresis range supports

applications where a bias winding on the flyback trans-

former is used to increase the efficiency of the LTC4267-1

switching regulator.

The I

/RUN can be driven below V

ITHSHDN

(nominally 0.28V

with respect to PGND) to force the LTC4267-1 switching

regulator into shutdown. An internal 0.3µA current source

always tries to pull the I

/RUN pin towards P

VCC

. When

the I

/RUN pin voltage is allowed to exceed V

ITHSHDN

and

VCC

exceeds V

TURNON

, the LTC4267-1 switching regulator

begins to operate and an internal clamp immediately pulls

the I

/RUN pin to about 0.7V. In operation, the I

/RUN

pin voltage will vary from roughly 0.7V to 1.9V to represent

current comparator thresholds from zero

to maximum

Internal Soft-Start

An internal soft-start feature is enabled whenever the

LTC4267-1 switching regulator comes out of shutdown.

Specifically, the I

/RUN voltage is clamped and is pre-

vented from reaching maximum until 1.4ms have passed.

This allows the input current of the PD to rise in a smooth

and controlled manner on start-up and stay within the cur-

rent limit requirement of the LTC4267-1 interface.

Adjustable Slope Compensation

The LTC4267-1 switching regulator injects a 5µA peak

current ramp out through its SENSE pin which can be

used for slope compensation in designs that require it.

This current ramp is approximately linear and begins at

zero current at 6% duty cycle, reaching peak current at

80% duty cycle. Programming the slope compensation

via a series resistor is discussed in the External Interface

and Component Selection section.

EXTERNAL INTERFACE AND COMPONENT SELECTION

Input Interface Transformer

Nodes on an Ethernet network commonly interface to the

outside world via an isolation transformer (Figure 9). For

PoE devices, the isolation transformer must include a

center tap on the media (cable) side. Proper termination

is required around the transformer to provide correct

impedance matching and to avoid radiated and conducted

emissions. Transformer vendors

such as Bel Fuse, Coil-

craft,

Pulse and Tyco (Table 3) can provide assistance with

selection of an appropriate isolation transformer and proper

termination methods. These vendors have transformers

specifically designed for use in PD applications.

Table 3. Power over Ethernet Transformer Vendors

VENDOR CONTACT INFORMATION

Bel Fuse Inc. 206 Van Vorst Street

Jersey City, NJ 07302

Tel: 201-432-0463

FAX: 201-432-9542

http://www.belfuse.com

Coilcraft, Inc. 1102 Silver Lake Road

Cary, IL 60013

Tel: 847-639-6400

FAX: 847-639-1469

http://www.coilcraft.com

Pulse Engineering 12220 World Trade Drive

San Diego, CA 92128

Tel: 858-674-8100

FAX: 858-674-8262

http://www.pulseeng.com

Tyco Electronics 308 Constitution Drive

Menlo Park, CA 94025-1164

Tel: 800-227-7040

FAX: 650-361-2508

http://www.circuitprotection.com

Figure 8. LTC4267-1 Switching Regulator

Start-Up/Shutdown State Diagram

LTC4267-1

PWM

SHUTDOWN

LTC4267-1

PWM

ENABLED

ITH

/RUN

< V

ITHSHDN

(NOMINALLY

0.28V)

ITH/RUN

> V

ITHSHDN

AND P

VCC

> V

TURNON

(NOMINALLY 8.7V)

VCC

< V

TURNOFF

42671 F08

ALL VOLTAGES WITH

RESPECT TO PGND

LTC4267-1

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Diode Bridge

IEEE 802.3af allows power wiring in either of two configu-

rations: on the TX/RX wires or via the spare wire pairs in

the RJ45 connector. The PD is required to accept power in

either polarity on either the main or spare inputs; therefore

it is common to install diode bridges on both inputs in

order to accommodate the different wiring configurations.

Figure 9 demonstrates an implementation of these diode

bridges. The IEEE 802.3af specification also mandates

that the leakage back through the unused bridge be less

than 28µA when the PD is powered with 57V.

The LTC4267-1 has several different modes of operation

based on the voltage present between V

PORTN

and V

PORTP

pins. The forward voltage drop of the input diodes in a

PD design subtracts from the input voltage and will af-

fect the transition point between modes. When using the

LTC4267-1, it is necessary to pay close attention to this

forward voltage drop. Selection of oversized diodes will help

keep the PD thresholds from exceeding IEEE specifications.

The input diode bridge of a PD can consume over 4% of

the available power in some applications. It may be de-

sirable to use Schottky

diodes in order to reduce power

loss. However, if the standard diode bridge is replaced

with a Schottky bridge, the transition points between the

modes will be affected. Figure 10 shows a technique for

using Schottky diodes while maintaining proper threshold

points to meet IEEE 802.3af compliance. D13 is added

to compensate for the change in UVLO turn-on voltage

caused by the Schottky diodes and consumes little power.

Input Capacitor

The IEEE 802.3af/at standard includes an impedance

requirement in order to implement the AC disconnect

function. A 0.1µF capacitor (C14 in Figure 9) is used to

meet the AC impedance requirement.

Input Series Resistance

Linear Technology has seen the customer community cable

discharge requirements increase by nearly 500,000 times

the original test levels. The PD must survive and operate

reliably not only when an initially charged cable connects

and dissipates the energy through the PD front end, but

also when the electrical power system grounds are subject

to very high energy events (e.g. lightning strikes).

In these high energy events, adding 10Ω series resistance

into the V

PORTP

pin greatly improves the robustness of

the LTC4267-1 based PD. (See Figure 9.) The TVS limits

the voltage across the port while the 10Ω

and

0.1µF ca-

pacitance reduces the edge rate the LT4267-1 encounters

across its pin. The added 10Ω series resistance does not

operationally affect the LTC4267-1 PD Interface nor does

it affect its compliance with the IEEE802.3 standard.

Figure 9. PD Front End with Isolation Transformer, Diode Bridges and Capacitor

–

RJ45

PULSE H2019

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SMAJ58A

TVS

BR1

HD01

BR2

HD01

TO PHY

SPARE

–

SPARE

C14

0.1µF

100V

LTC4267-1

10Ω

PORTN

PORTP

LTC4267-1

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Transient Voltage Suppressor

The LTC4267-1 specifies an absolute maximum voltage of

100V and is designed to tolerate brief overvoltage events.

However, the pins that interface to the outside world can

routinely see excessive peak voltages. To protect the

LTC4267-1, install a transient voltage suppressor (D3)

between the input diode bridge and the LTC4267-1 as

shown in Figure 9. A SMAJ58A is recommended for typical

PD applications. However, a SMBJ58A may be preferred in

applications where the PD front-end must absorb higher

energy discharge events.

Classification Resistor Selection (R

CLASS

)

The IEEE specification allows classifying PDs into four

distinct classes with class 4 being reserved for future use

(Table 2). An external resistor connected from R

CLASS

PORTN

(Figure 4) sets the value of the load current. The

designer should determine which power category the PD

falls into and then select the appropriate value of R

CLASS

from Table 2. If a unique load current is required, the value

of R

CLASS

can be calculated as:

CLASS

= 1.237V/(I

DESIRED

– I

IN_CLASS

)

where I

IN_CLASS

is the LTC4267-1 IC supply current during

classification and is given in the electrical specifications.

The R

CLASS

resistor must be 1% or better to avoid de-

grading the overall accuracy of the classification circuit.

Resistor power dissipation will be 50mW maximum and

is transient so heating is typically not a concern. In order

to maintain loop stability, the layout should minimize

capacitance at the R

CLASS

node. The classification circuit

Figure 10. PD Front End with Isolation Transformer, Tw o Schottky Diode Bridges

–

SPARE

–

FROM

PSE

RJ45

TXOUT

OUT

TO PHY

TXOUT

–

SPARE

RXOUT

–

NOTES: UNLESS OTHERWISE SPECIFIED

1. ALL RESISTORS ARE 5%

2. SELECT R

CLASS

FOR CLASS 1-4 OPERATION. REFER

TO DATA SHEET APPLICATIONS INFORMATION SECTION

C2: AVX 1808GC102MAT

D9 TO D12, D14 TO D17: DIODES INC., B1100

T1: PULSE H2019

CLASS

PORTN

D13

MMSD4148

C11

0.1µF

100V

SMAJ58A

R30

75Ω

C24

0.01µF

200V

R31

75Ω

C25

0.01µF

200V

75Ω

0.01µF

200V

75Ω

0.01µF

200V

1000pF

2kV

D10

B1100

D12

B1100

10Ω

B1100

D11

B1100

D17

B1100

D16

B1100

D15

B1100

D14

B1100

PORTP

LTC4267-1

42671 F10

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P32

LTC4267CGN-1#TRPBF

Mfr. #:

Buy LTC4267CGN-1#TRPBF

Manufacturer:

Analog Devices Inc.

Description:

Power Switch ICs - POE / LAN IEEE802.3af PD w/Switching Reg.

Lifecycle:

New from this manufacturer.

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LTC4267CGN-1#TRPBF

LTC4267CGN-1#TRPBF

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