LTC4257IS8 Linear Technology, LTC4257IS8 Datasheet - Page 3

• Manufacturer Part Number: LTC4257IS8
• Description: IC CONTROLLER POE INTERFAC 8SOIC
• Manufacturer: Linear Technology
• Type: Power over Ethernet Switch (PoE)r

Specifications of LTC4257IS8

• Applications: IP Phones, Power over LAN, Network Routers and Switches
• Internal Switch(s): Yes
• Current Limit: 350mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All voltages are with respect to GND pin.
Note 3: The LTC4257 operates with a negative supply voltage in the range
of –1.5V to –57V. To avoid confusion, voltages in this data sheet are
always referred to in terms of absolute magnitude. Terms such as
“maximum negative voltage” refer to the largest negative voltage and a
“rising negative voltage” refers to a voltage that is becoming more
negative.
Note 4: The LTC4257 is designed to work with two polarity protection
diodes between the PSE and PD. Parameter ranges specified in the
Electrical Characteristics are with respect to LTC4257 pins and are
designed to meet IEEE 802.3af specifications when these diode drops are
included. See Applications Information.
Note 5: Signature resistance is measured via the 2-point ∆V/∆I method as
defined by IEEE 802.3af. The LTC4257 signature resistance is offset from
25k to account for diode resistance. With two series diodes, the total PD
resistance will be between 23.75k and 26.25k and meet IEEE 802.3af
specifications. The minimum probe voltages measured at the LTC4257
pins are –1.5V and –2.5V. The maximum probe voltages are –8.5V and
–9.5V.
Note 6: The LTC4257 includes hysteresis in the UVLO voltages to preclude
any start-up oscillation. Per IEEE 802.3af requirements, the LTC4257 will
temperature range, otherwise specifications are at T
SYMBOL
R
V
V
V
I
R
I
I
I
T
T
PG_LEAK
OUT_LEAK
LIMIT
LIMIT_WARM
OVERTEMP
SHUTDOWN
PG_OUT
PG_THRES_FALL
PG_THRES_RISE
SIGNATURE
ON
PARAMETER
Signature Resistance
Power Good Output Low Voltage
Power Good Trip Point
Power Good Leakage
On-Resistance
V
Input Current Limit
Overtemperature Input Current Limit
Overtemperature Trip Temperature
Thermal Shutdown Trip Temperature
OUT
Leakage
CONDITIONS
–1.5V ≤ V
IEEE 802.3af 2-Point Measurement (Notes 4, 5)
I = 1mA, V
V
V
I = 300mA, V
(Note 9)
V
V
(Note 11)
(Note 11)
(Note 11)
IN
IN
IN
IN
A
V
V
OUT
OUT
= 25°C. (Note 3)
= –48V, Voltage Between V
= 0V, PWRGD FET Off, V
= 0V, Power MOSFET Off, V
= – 48V, V
Falling
Rising
The
IN
IN
≤ –9.5V, V
= – 48V, PWRGD Referenced to V
IN
●
OUT
= – 48V, Measured from V
denotes the specifications which apply over the full operating
= –43V (Note 11)
power up from a voltage source with 20Ω series resistance on the first
trial.
Note 7: I
Pin 2. Total supply current in classification mode will be I
(see Note 8).
Note 8: I
∆I
I
include variations in R
a PD also includes the IC quiescent current (I
Information.
Note 9: For the DD package, this parameter is assured by design and
wafer level testing.
Note 10: I
good status circuit. This current is compensated for in the 25kΩ signature
resistance and does not affect PD operation.
Note 11: The LTC4257 includes smart thermal protection. In the event of
an overtemperature condition, the LTC4257 will reduce the input current
limit by 50% to reduce the power dissipation in the package. If the part
continues heating and reaches the shutdown temperature, the current is
reduced to zero until the part cools below the overtemperature limit. The
LTC4257 is also protected against thermal damage from incorrect
classification probing by the PSE. If the LTC4257 exceeds the
overtemperature trip point, the classification load current is disabled.
CLASS
OUT
CLASS
Tied to GND,
= 1.237/R
PWRGD
accuracy is with respect to the ideal current defined as
IN_CLASS
CLASS
IN
OUT_LEAK
OUT
and V
= 57V
= 57V (Note 10)
is the measured current flowing through R
CLASS
OUT
does not include classification current programmed at
includes current drawn at the V
IN
(Note 9)
. The current accuracy specification does not
CLASS
to V
IN
OUT
resistance. The total classification current for
●
●
●
●
●
●
●
●
23.25
MIN
300
1.3
2.7
IN_CLASS
TYP
350
188
120
140
1.5
3.0
1.0
OUT
LTC4257
). See Applications
pin by the power
IN_CLASS
CLASS
26.00
MAX
150
400
0.5
1.7
3.3
1.6
2.0
1
.
+ I
UNITS
CLASS
4257fb
3
mA
mA
kΩ
µA
µA
°C
°C
Ω
Ω
V
V
V