LT4295 datasheet(24/50 Pages) AD

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LTC9101-1/

LTC9102/LTC9103

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

for the connected device to be categorized as a single-

signature PD, a dual-signature PD, or an invalid result.

An invalid connection check result is reported when a

device is added or removed during connection check.

Connection check only affects operation in 4-pair mode.

In 4-pair mode a detection cycle always includes a con-

nection check unless the port is in AT mode. See Figure 1.

In 2-pair mode connection check is not enabled (intrinsi-

cally a 4-pair only operation) and all PDs are reported

as single-signature regardless of the PD’s actual

signature configuration.

DETECTION

Detection Overview

To avoid damaging network devices that were not designed

to tolerate DC voltage, a PSE must determine whether the

connected device is a valid PD before applying power.

The IEEE 802.3 specification requires that a valid PD has

a common-mode resistance of 25k ±5% at any channel

voltage below 10V. The PSE must accept resistances that

fall between 19k and 26.5k, and it must reject resistances

above 33k or below 15k (shaded regions in Figure 13).

The PSE may choose to accept or reject resistances in

the undefined areas between the must-accept and must-

reject ranges. In particular, the PSE must reject standard

computer Network Interface Cards (NICs), many of which

have 150Ω common-mode termination resistors that will

be damaged if power is applied to them (the black region

at the left of Figure 13).

Figure 13.

RESISTANCE

PSE

10k

15k

429291 F13

19k

26.5k

26.25k

23.75k

150 (NIC)

20k

30k

33k

IEEE 802.3 Signature Resistance Ranges

Table 7. Typical Auto Mode Power On Thresholds,

Dual-Signature PD**

PSE ASSIGNED

CLASS

PER-CHANNEL

ICUT-2P

ILIM-2P

PCUT-2P*

94mA

425mA

4.86W

150mA

425mA

7.56W

338mA

425mA

16.2W

638mA

850mA

32.4W

975mA

1167mA

48.1W

*A per-port PCUT-4P threshold holds the sum of PCUT-2P for each

powered channel.

**RSENSE = 0.1Ω.

CONNECTION CHECK

Connection Check Overview

IEEE 802.3bt introduces a new detection subroutine known

as connection check. A connection check is required to

determine whether the attached PD is a single-signature

PD, a dual-signature PD or an invalid result.

In 802.3at, only one PD configuration was described;

this is known as a single-signature PD and is shown in

Figure 11. A single-signature PD presents the same 25k

detection resistor to both the pairsets in parallel.

New in 802.3bt is the dual-signature PD as shown

in Figure 12. A dual-signature PD presents two fully

independent 25k detection signature resistors, one to

each pairset.

The PD configuration (single or dual) determines how

the PD is managed during subsequent detection, classi-

fication and power on procedures. Throughout this data

sheet attention will be called to the different treatment of

single-signature and dual-signature PDs.

Connection check is performed with two current measure-

ments, at the same forced voltage, on the first channel.

The second channel is tested for aggressor behavior by

introducing a forced current on the second channel during

the second measurement. Comparison of the two result-

ing current measurements on the first channel allows

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