LTC4257
IEEE 802.3af PD
Power over Ethernet
Interface Controller
FEATURES
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DESCRIPTIO
Complete Power Interface Port for IEEE 802.3af
®
Powered Devices (PDs)
Onboard 100V, 400mA Power MOSFET
Precision Input Current Limit
Onboard 25k Signature Resistor
Programmable Classification Current (Class 0-4)
Undervoltage Lockout
Smart Thermal Protection
Power Good Signal
Available in 8-Pin SO and Low Profile (3mm
×
3mm)
DFN Packages
The LTC
®
4257 provides complete signature and power
interface functions for a device operating in an IEEE
802.3af Power over Ethernet (PoE) system. The LTC4257
simplifies Powered Device (PD) design by incorporating
the 25k signature resistor, the classification current source,
input current limit with thermal foldback, undervoltage
lockout and power good signalling, all in a single 8-pin
package. By incorporating a high voltage power MOSFET
onboard, the LTC4257 provides the system designer with
reduced cost while also saving board space.
The LTC4257 can interface directly with a variety of Linear
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
solutions for Power Sourcing Equipment (PSE) applica-
tions with quad network power controllers.
The LTC4257 is available in the 8-pin SO and low profile
(3mm
×
3mm) DFN packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
APPLICATIO S
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IP Phone Power Management
Wireless Access Points
Telecom Power Control
TYPICAL APPLICATIO
Powered Device (PD)
–48V FROM
POWER SOURCING
EQUIPMENT
(PSE)
~
+
0.1µF
SMAJ58A
LTC4257
GND
R
CLASS
100k
PWRGD
V
IN
V
OUT
5µF
MIN
V
IN
+
DF01SA
SWITCHING
POWER SUPPLY
SHDN
RTN
+
3.3V
TO LOGIC
~
–
R
CLASS
4257 TA01
–
PWRGD
50V/DIV
5ms/DIV
4225 TA02
U
LTC4257 Charging 300µF
Load Capacitor
V
IN
50V/DIV
V
OUT
20V/DIV
I
IN
200mA/DIV
4257fb
U
U
1
LINER [ LINEAR TECHNOLOGY ]
