LTC4257-1

1

42571fa

The LTC

interface functions for a device operating in an IEEE

802.3af Power over Ethernet (PoE) system. The LTC4257-1

simplifies Powered Device (PD) design by incorporating

the 25k signature resistor, classification current source,

input current limit, undervoltage lockout, thermal over-

load protection, signature disable and power good signal-

ling, all in a single 8-pin package. The LTC4257-1 includes

a precision, dual level current limit circuit. This allows it to

charge large load capacitors and interface with legacy

Power over Ethernet systems while maintaining compat-

ibility with the current IEEE 802.3af specification. By

incorporating a high voltage power MOSFET onboard, the

LTC4257-1 provides the system designer with reduced

cost while also saving board space.

The LTC4257-1 can interface directly with a variety of Lin-

ear Technology DC/DC converter products to provide a cost

effective power solution for IP phones, wireless access

points and other PDs. Linear Technology also provides

network power controllers for Power Sourcing Equipment

(PSE) applications.

The LTC4257-1 is available in the 8-pin SO and low profile

(3mm

× 3mm) DFN packages.

s

IP Phone Power Management

s

Wireless Access Points

s

Telecom Power Control

, LTC and LT are registered trademarks of Linear Technology Corporation.

s

Complete Power Interface Port for IEEE 802

Powered Devices (PDs)

s

Onboard 100V, 400mA Power MOSFET

s

Precision Dual Level Current Limit

s

Onboard 25k Signature Resistor with Disable

s

Programmable Classification Current (Class 0-4)

s

Undervoltage Lockout

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Thermal Overload Protection

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Power Good Signal

s

Available in 8-Pin SO and Low Profile (3mm

× 3mm)

DFN Packages

IEEE 802.3af PD

Power over Ethernet Interface

Controller with Dual Current Limit

GND

SMAJ58A

0.1

µF

SIGDISA

PWRGD

LTC4257-1

SHDN

100k

5

µF

MIN

3.3V

TO LOGIC

RTN

SWITCHING

POWER SUPPLY

–48V FROM

POWER SOURCING

EQUIPMENT

(PSE)

42571 TA01

+

–

+

DF01SA

~

~

+

–

Powered Device (PD)

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATIO

802 is a registered trademark of Institute of Electrical and Electronics Engineers, Inc.