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LTC4278

4278fc

APPLICATIONS INFORMATION

To control the power-on surge currents in the system, the

LTC4278 provides a fixed inrush current, allowing C1 to

ramp up to the line voltage in a controlled manner.

The LTC4278 keeps the PD inrush current below the PSE

current limit to provide a well controlled power-up charac-

teristicthatisindependentofthePSEbehavior.Thisensures

a PD using the LTC4278 interoperability with any PSE.

TURN-ON/ TURN-OFF THRESHOLD

The IEEE 802.3af/at specification for the PD dictates a

maximum turn-on voltage of 42V and a minimum turn-off

voltage of 30V. This specification provides an adequate

voltage to begin PD operation, and to discontinue PD

operation when the input voltage is too low. In addition,

this specification allows PD designs to incorporate an ON/

OFF hysteresis window to prevent start-up oscillations.

The LTC4278 features an ON/OFF hysteresis window (see

Figure 5) that conforms with the IEEE 802.3af/at specifi-

cation and accommodates the voltage drop in the cable

and input diode bridge at the onset of the inrush current.

Once C1 is fully charged, the LTC4278 turns on is internal

MOSFET and passes power to the PD load. The LTC4278

continues to power the PD load as long as the input voltage

does not fall below the OFF threshold. When the LTC4278

input voltage falls below the OFF threshold, the PD load

Figure 5. LTC4278 ON/OFF and Overvoltage Lockout

VPORTP

5µF

MIN

VPORTN

VNEG

LTC4278

4278 F05

PSE

ON/OFF AND

OVERVOLTAGE

LOCKOUT

CIRCUIT

LOAD

CURRENT-LIMITED

TURN ON

VPORTP – VPORTN

LTC4278

VOLTAGE

POWER MOSFET

0V TO ON*

OFF

>ON*

<OFF*

OFF

>OVLO

OFF

*INCLUDES ON/OFF HYSTERESIS

ON THRESHOLD ≅ 36.1V

OFF THRESHOLD ≅ 30.7V

OVLO THRESHOLD ≅ 71.0V

Figure 6. LTC4278 Power Good Functional and State Diagram

4278 F06

BOLD LINE INDICATES HIGH CURRENT PATH

PWRGD

POWER

NOT

GOOD

INRUSH COMPLETE

ON < VPORTP < OVLO

AND NOT IN THERMAL SHUTDOWN

VPORTP < OFF

VPORTP > OVLO

OR THERMAL SHUTDOWN

POWER

GOOD

PWRGD

LTC4278

VNEG

VPORTN 6

VPORTN

OVLO

ON/OFF

TSD

CONTROL

CIRCUIT

is disconnected, and classification mode resumes. C1

discharges through the LTC4278 circuitry.

COMPLEMENTARY POWER GOOD

WhenLTC4278fullychargestheloadcapacitor(C1),power

good is declared and the LTC4278 load can safely begin

operation. The LTC4278 provides complementary power

good signals that remain active during normal operation

and are de-asserted when the input voltage falls below

the OFF threshold, when the input voltage exceeds the

overvoltage lockout (OVLO) threshold, or in the event of

a thermal shutdown (see Figure 6).

The PWRGD pin features an open collector output refer-

enced to VNEG which can interface directly with the UVLO

pin. When power good is declared and active, the PWRGD

pinishighimpedancewithrespecttoVNEG.Aninternal14V

clamp protects the UVLO pin from an excessive voltage.

The active low PWRGD pin connects to an internal, open-

drain MOSFET referenced to VPORTN and may be used as

an indicator bit when power good is declared and active.

The PWRGD pin is low impedance with respect to VPORTN.

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