LTC4257

3

4257fb

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

The

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

power up from a voltage source with 20

Ω series resistance on the first

trial.

(see Note 8).

Information.

Note 9: For the DD package, this parameter is assured by design and

wafer level testing.

good status circuit. This current is compensated for in the 25k

Ω 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.

ELECTRICAL CHARACTERISTICS

Signature Resistance

–1.5V

●

23.25

26.00

k

Ω

IEEE 802.3af 2-Point Measurement (Notes 4, 5)

Power Good Output Low Voltage

●

0.5

V

Power Good Trip Point

●

1.3

1.5

1.7

V

●

2.7

3.0

3.3

V

Power Good Leakage

●

1

µA

On-Resistance

1.0

1.6

Ω

(Note 9)

●

2.0

Ω

●

150

µA

Input Current Limit

●

300

350

400

mA

Overtemperature Input Current Limit

(Note 11)

188

mA

Overtemperature Trip Temperature

(Note 11)

120

°C

Thermal Shutdown Trip Temperature

(Note 11)

140

°C