Datasheet

LTC4257-1
10
42571fb
Undervoltage Lockout
The IEEE specification dictates a maximum turn-on volt-
age of 42V and a minimum turn-off voltage of 30V for the
PD. In addition, the PD must maintain large on-off hyster-
esis to prevent resistive losses in the wiring between the
PSE and the PD from causing start-up oscillation. The
LTC4257-1 incorporates an undervoltage lockout (UVLO)
circuit that monitors line voltage to determine when to
apply power to the PD load (Figure 6). Before power is
applied to the load, the V
OUT
pin is high impedance and
sitting at ground potential since there is no charge on
capacitor C1. When the input voltage rises above the UVLO
turn-on threshold, the LTC4257-1 removes the classifica-
tion load current and turns on the internal power MOSFET.
C1 charges up under LTC4257-1 current limit control and
the V
OUT
pin transitions from 0V to V
IN
. This sequence is
shown in Figure 1. The LTC4257-1 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 LTC4257-1 integrates a dual level current limit circuit
with an onboard power MOSFET and sense resistor to
provide a complete inrush control circuit without additional
external components. At turn on, the LTC4257-1 will limit
input current to the low level, allowing the load capacitor
to ramp up to the line voltage in a controlled manner.
The LTC4257-1 has been specifically designed to interface
with legacy PSEs which do not meet the inrush current
requirement of the IEEE 802.3af specification. This is
APPLICATIO S I FOR ATIO
WUUU
GND
C1
5µF
MIN
V
IN
8
4
V
OUT
5
LTC4257-1
42571 F06
TO
PSE
UNDERVOLTAGE
LOCKOUT
CIRCUIT
PD
LOAD
CURRENT-LIMITED
TURN ON
+
INPUT LTC4257-1
VOLTAGE POWER MOSFET
0V TO UVLO* OFF
>UVLO* ON
*UVLO INCLUDES HYSTERESIS
RISING INPUT THRESHOLD ≅ –36V
FALLING INPUT THRESHOLD ≅ –30.5V
Figure 6. LTC4257-1 Undervoltage Lockout