LT4294 datasheet(19/40 Pages) AD

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LTC4291-1/LTC4292

Rev 0

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Auto mode operates the same as semi-auto mode except

it will automatically apply power to the port if detec-

tion and classification are successful. Auto mode will

autonomously set the ICUT-2P, ILIM-2P, and PCUT-4P values

based on the Class result. This operational mode may

be entered by setting AUTO high at reset or by changing

the OPMD state to Auto. See Auto Mode Maximum PSE

Power section.

In shutdown mode the port is disabled and will not detect

or power a PD.

Regardless of which mode it is in, the LTC4291-1/

LTC4292 will remove power automatically from any port

and/or channel, as appropriate, that generates a fault. It

will also automatically remove power from any port/chan-

nel that generates a disconnect event if disconnect detec-

tion is enabled. The host controller may also command

the port to remove power at any time.

Reset and the AUTO Pin

The initial LTC4291-1/LTC4292 configuration depends on

the state of AUTO during reset. Reset occurs at power-up,

whenever RESET is pulled low, or when the global Reset

All bit is set. Changing the state of AUTO after power-

up will not change the port behavior of the LTC4291-1/

LTC4292 until a reset occurs.

Although typically actively managed by a host controller,

the LTC4291-1/LTC4292 may alternatively be configured

for autonomous operation by setting AUTO high. With

AUTO high, each port will detect and classify repeatedly

until a PD is discovered, set ICUT-2P, ILIM-2P, and PCUT-4P

according to the PSE assigned Class, apply power to valid

PDs, and remove power when a PD is disconnected.

Tables 4 and 5 show the ICUT-2P, ILIM-2P, and PCUT-4P val-

ues that will be automatically set in auto mode, based on

the PD requested Class.

Table 4. Typical Auto Mode Power On Thresholds,

Single-Signature PD

CLASS

PER-CHANNEL

PER-PORT

ICUT-2P

ILIM-2P

PCUT-4P

94mA

425mA

5.43W

150mA

425mA

8.69W

338mA

425mA

19.5W

638mA

850mA

36.4W

581mA

850mA

52.7W

731mA

850mA

70.0W

825mA

1063mA

87.4W

975mA

1167mA

96.6W

Table 5. Typical Auto Mode Power On Thresholds,

Dual-Signature PD

CLASS

PER-CHANNEL

ICUT-2P

ILIM-2P

PCUT-2P*

94mA

425mA

5.43W

150mA

425mA

8.69W

338mA

425mA

19.5W

638mA

850mA

36.4W

975mA

1167mA

48.3W

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

powered channel.

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 10. 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 11. A dual-signature PD presents two fully indepen-

dent 25k detection signature resistors, one to each pairset.

APPLICATIONS INFORMATION

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