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LTC4265

4265f

APPLICATIONS INFORMATION

In 2-event classiﬁcation, a Type-2 PSE probes for power

classiﬁcation twice. Figure 4 presents an example of a

2-event classiﬁcation. The 1st classiﬁcation event occurs

when the PSE presents an input voltage between 14.5V to

20.5V and the LTC4265 presents a class 4 load current.

The PSE then drops the input voltage into the mark volt-

age range of 6.9V to 10V, signaling the 1st mark event.

The PD in the mark voltage range presents a load current

between 0.25mA to 4mA.

The PSE repeats this sequence, signaling the 2nd Clas-

siﬁcation and 2nd mark event occurrence. This alerts the

LTC4265 that a Type-2 PSE is present. The Type-2 PSE

then applies power to the PD and the LTC4265 charges

up the reservoir capacitor C1 with a controlled inrush cur-

rent. When C1 is fully charged, and the LTC4265 declares

power good, the T2PSE pin presents an active low signal,

or low impedance output with respect to VIN. The T2PSE

output becomes inactive when the LTC4265 input voltage

falls outside the normal operating range.

SIGNATURE CORRUPT DURING MARK

As a member of the IEEE802.3at working group, Linear

notes 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 possibility by

requiring a Type-2 PD to corrupt the signature resistance

during the mark event, alerting the PSE not to apply power.

The LTC4265 conforms to this standard by internally

corrupting the signature resistance. This also discharges

the port before the PSE begins the next detection cycle.

DETECTION V1

UVLO

= RLOAD C1

TRACKS

VIN

DETECTION V2

TIME

40mA

TIME

–10

TIME

–20

–30

–40

–50

INRUSH

4265 F04

INRUSH = 100mA

RCLASS = 30.9Ω

ILOAD =

VIN

RLOAD

GND

PSE

IIN

RLOAD

RCLASS

VOUT

GND

RCLASS

T2PSE

LTC4265

VOUT

VIN

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. VOUT, T2PSE, and PD Current

as a Result of 2-event Classiﬁcation

LTC4265 Datasheet(PDF) 9 Page - Linear Technology

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