LTC4258 datasheet(16/42 Pages) LINER

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LTC4278

4278fc

APPLICATIONS INFORMATION

DETECTION V1

OFF

τ = RLOAD C1

TRACKS

VPORTN

DETECTION V2

TIME

40mA

TIME

–10

TIME

–20

–30

–40

–50

INRUSH

4278 F04

INRUSH = 100mA

RCLASS = 30.9Ω

ILOAD =

VPORTN

RLOAD

VPORTP

PSE

IIN

RLOAD

RCLASS

T2P

LTC4278

VNEG

VPORTN

1st CLASS

1st MARK 2nd MARK

DETECTION V1

DETECTION V2

1st MARK 2nd MARK

2nd CLASS

1st CLASS

2nd CLASS

LOAD, ILOAD

INRUSH

Figure 4. VNEG, T2P and PD Current

as a Result of 2-Event Classification

SIGNATURE CORRUPT DURING MARK

As a member of the IEEE 802.3at working group, Linear

Technology noted that it is possible for a Type 2 PD to

receive a false indication of a 2-event classification if a

PSE port is pre-charged to a voltage above the detection

voltage range before the first detection cycle. The IEEE

working group modified the standard to prevent this pos-

sibility by requiring a Type 2 PD to corrupt the signature

resistance during the mark event, alerting the PSE not to

apply power. The LTC4278 conforms to this standard by

corrupting the signature resistance. This also discharges

the port before the PSE begins the next detection cycle.

PD STABILITY DURING CLASSIFICATION

Classificationpresentsachallengingstabilityproblemdue

to the wide range of possible classification load current.

The onset of the classification load current introduces a

voltage drop across the cable and increases the forward

voltage of the input diode bridge. This may cause the PD

to oscillate between detection and classification with the

onset and removal of the classification load current.

The LTC4278 prevents this oscillation by introducing a

voltagehysteresiswindowbetweenthedetectionandclas-

sification ranges. The hysteresis window accommodates

the voltage changes a PD encounters at the onset of the

classification load current, thus providing a trouble-free

transition between detection and classification modes.

TheLTC4278alsomaintainsapositiveI-Vslopethroughout

the classification range up to the on-voltage. In the event

a PSE overshoots beyond the classification voltage range,

the available load current aids in returning the PD back

into the classification voltage range. (The PD input may

otherwise be “trapped” by a reverse-biased diode bridge

and the voltage held by the 0.1μF capacitor).

INRUSH CURRENT

Once the PSE detects and optionally classifies the PD,

the PSE then applies powers on the PD. When the

LTC4278inputvoltagerisesabovetheon-voltagethreshold,

LTC4278 connects VNEG to VPORTN through the internal

power MOSFET.

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