LTC4267IGN datasheet(16/32 Pages) LINER

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LTC4267

4267f

important not to confuse the undervoltage lockout of the

PD interface at VPORTN with that of the switching regulator

at PVCC. They are independent functions.

prevented 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 current limit requirement of the LTC4267 interface.

Adjustable Slope Compensation

The LTC4267 switching regulator injects a 5µA peak cur-

rent 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 Pulse, Bel Fuse,

Tyco and others (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

Pulse Engineering

12220 World Trade Drive

San Diego, CA 92128

Tel: 858-674-8100

FAX: 858-674-8262

http://www.pulseeng.com

Bel Fuse Inc.

206 Van Vorst Street

Jersey City, NJ 07302

Tel: 201-432-0463

FAX: 201-432-9542

http://www.belfuse.com

Tyco Electronics

308 Constitution Drive

Menlo Park, CA 94025-1164

Tel: 800-227-7040

FAX: 650-361-2508

http://www.circuitprotection.com

APPLICATIO S I FOR ATIO

Figure 8. LTC4267 Switching Regulator

Start-Up/Shutdown State Diagram

The undervoltage lockout mechanism on PVCC prevents

the LTC4267 switching regulator from trying to drive the

external N-Channel MOSFET with insufﬁcient gate-to-

source voltage. The voltage at the PVCC pin must exceed

VTURNON (nominally 8.7V with respect to PGND) at least

momentarily to enable operation. The PVCC voltage must

fall to VTURNOFF (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

switching regulator.

The ITH/RUN can be driven below VITHSHDN (nominally

0.28V with respect to PGND) to force the LTC4267 switching

regulator into shutdown. An internal 0.3µA current source

always tries to pull the ITH/RUN pin towards PVCC. When

the ITH/RUN pin voltage is allowed to exceed VITHSHDN and

PVCC exceeds VTURNON, the LTC4267 switching regulator

begins to operate and an internal clamp immediately pulls

the ITH/RUN pin to about 0.7V. In operation, the ITH/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 switching regulator comes out of shutdown.

Speciﬁcally, the ITH/RUN voltage is clamped and is

LTC4267

PWM

SHUTDOWN

LTC4267

PWM

ENABLED

VITH/RUN

< VITHSHDN

(NOMINALLY

0.28V)

VITH/RUN > VITHSHDN

AND PVCC > VTURNON

(NOMINALLY 8.7V)

PVCC < VTURNOFF

4267 F08

ALL VOLTAGES WITH

RESPECT TO PGND

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