LTC4257-1 Linear Technology, LTC4257-1 Datasheet - Page 5

• Manufacturer Part Number: LTC4257-1
• Description: Power over Ethernet Interface Controller with Dual Current Limit
• Manufacturer: Linear Technology

BLOCK DIAGRA
PI FU CTIO S
NC (Pin 1): No Internal Connection.
R
value the LTC4257-1 maintains during classification. Con-
nect a resistor between R
NC (Pin 3): No Internal Connection.
V
input diode bridge.
V
through an internal power MOSFET that limits input cur-
rent. V
above the turn-on UVLO threshold. The output is then
current limited. See Applications Information.
IN
OUT
CLASS
U
(Pin 4): Power Input. Tie to system – 48V through the
(Pin 5): Power Output. Supplies – 48V to the PD load
OUT
(Pin 2): Class Select Input. Used to set the current
U
is high impedance until the input voltage rises
R
CLASS
U
V
NC
NC 3
IN
1
2
4
CLASS
W
375mA
140mA
CONTROL
CIRCUITS
and V
0.3
1.237V
CLASSIFICATION
CURRENT LOAD
IN
(see Table 2).
+
–
+
–
EN
EN
INPUT
CURRENT
LIMIT
25k SIGNATURE
BOLD LINE INDICATES HIGH CURRENT PATH
PWRGD (Pin 6): Power Good Output, Open-Drain. Signals
to the PD load that the LTC4257-1 MOSFET is on and that
the PD’s DC/DC converter can start operation. Low imped-
ance indicates power is good. PWRGD is high impedance
during detection, classification and in the event of a
thermal overload. PWRGD is referenced to V
SIGDISA (Pin 7): Signature Disable Input. Allows the PD
to command the LTC4257-1 to present an invalid signa-
ture resistance and to remain inactive. Connecting SIGDISA
to GND lowers the signature resistance to an invalid value.
If left floating, SIGDISA is internally pulled to V
unused, tie SIGDISA to V
GND (Pin 8): Ground. Tied to system ground and power
return through the input diode bridge.
RESISTOR
POWER GOOD
9k
16k
SIGNATURE DISABLE
IN
.
8
7
6
5
42571 BD
V
GND
SIGDISA
PWRGD
OUT
LTC4257-1
IN
.
IN
42571fa
5
. If