Datasheet

LTC4257
9
4257fb
probing exceeds 75ms, the LTC4257 may overheat. In this
situation, the thermal protection circuit will engage and
disable the classification current source in order to protect
the part. The LTC4257 stays in classification mode until
the input voltage rises above the UVLO turn-on voltage.
Undervoltage Lockout
I EEE 802.3af dictates a maximum turn-on voltage of 42V
and a minimum turn-off voltage of 30V for the PD. In
addition, the PD must maintain large on-off hysteresis to
prevent resistive losses in the wiring between the PSE and
the PD from causing start-up oscillation. The LTC4257
incorporates an undervoltage lockout (UVLO) circuit that
monitors line voltage to determine when to apply power
to the PD load (Figure 5). Before power is applied to the
load, the V
OUT
pin is high impedance and at ground
potential since there is no charge on capacitor C1. When
the input voltage rises above the UVLO turn-on threshold,
the LTC4257 removes the classification load current and
turns on the internal power MOSFET. C1 charges up under
LTC4257 current limit control and the V
OUT
pin transitions
from 0V to V
IN
. This sequence is shown in Figure 1. The
LTC4257 includes a hysteretic UVLO circuit that keeps
power applied to the load until the input voltage falls
below the UVLO turn-off threshold. Once the input voltage
drops below –30V, the internal power MOSFET is turned
off and the classification load current is re-enabled.
C1 will discharge through the PD circuitry and the V
OUT
pin will go to a high impedance state.
Input Current Limit
IEEE 802.3af specifies a maximum inrush current and also
specifies a minimum load capacitor between the GND and
V
OUT
pins. To control turn-on surge current in the system,
the LTC4257 integrates a current limit circuit with the
onboard power MOSFET and sense resistor to provide a
complete inrush control circuit without additional external
components. The LTC4257 limits input current to less
than the 400mA maximum specified by 802.3af, allowing
the load capacitor to ramp up to the line voltage in a
controlled manner. During this ramp up, a large amount of
power is dissipated in the power MOSFET. The LTC4257
Figure 5. LTC4257 Undervoltage Lockout
APPLICATIO S I FOR ATIO
WUUU
GND
C1
5µF
MIN
V
IN
8
4
V
OUT
5
LTC4257
4257 F05
TO
PSE
UNDERVOLTAGE
LOCKOUT
CIRCUIT
PD
LOAD
CURRENT-LIMITED
TURN ON
+
INPUT LTC4257
VOLTAGE POWER MOSFET
0V TO UVLO* OFF
>UVLO* ON
*UVLO INCLUDES HYSTERESIS
RISING INPUT THRESHOLD ≅ –39.2V
FALLING INPUT THRESHOLD ≅ –30.5V