LTC4292 datasheet(18/40 Pages) AD

18 / 40 page

LTC4291-1/LTC4292

Rev 0

For more information www.analog.com

Table 1. IEEE-Specified Power Allocations, Single-Signature PD

PD CLASS

PSE OUTPUT

POWER

ALLOCATED

CABLING LOSS

PD INPUT

POWER

0.16W

3.84W

6.7W

0.21W

6.49W

14W

13W

30W

4.5W

25.5W

45W

40W

60W

51W

75W

13W

62W

90W

18.7W

71.3W

BACKWARD COMPATIBILITY

The LTC4291-1/LTC4292 may be configured as an

802.3bt-compliant PSE, either Type 3 or Type 4. While

802.3bt PSEs cannot identify as an 802.3at Type 1 or

Type 2 PSE, there is no loss in PSE functionality; all

802.3bt-compliant PSEs are fully backwards compatible

with existing 802.3at Type 1 and Type 2 PDs as shown

in Table 2. In addition to full compatibility, 802.3bt PSEs

extend support for lower standby power, enhanced cur-

rent limit timing, and dynamic power management to all

PD Types (as supported by the PD application).

Table 2. PSE Maximum Delivered Power, Per-Port

DEVICE

PSE

STANDARD

802.3at

802.3bt

TYPE

802.3at

13W

13W*

25.5W

802.3bt

13W*

25.5W*

51W

13W*

25.5W*

51W*

71.3W

*Indicates PD allocated less power than requested.

Software register map compatibility with LTC4266 and

LTC4271-based PSEs has been maintained to the extent

possible. LTC4291-based PSEs utilize two channels to

control a single PSE port. This multiplicity of channel

status and control requires extensions to the existing

For register map details please contact Analog Devices

to request the LTC4291 Software Programming

documentation.

Special Compatibility Mode Notes

As with prior generations, each I2C address provides

status and control for four PoE ports. Each port reg-

ister slice provides port control and status as well as

channel A vs B control and status.

Certain status registers, e.g. Port Status and Power

Status, relate to a channel state, as opposed to port

state and are split into three copies; a generalized port

state, channel A state and channel B state.

Certain command registers, e.g., Power-on pushbutton,

likewise are bifurcated to allow per-channel control.

OPERATING MODES

The LTC4291-1/LTC4292 includes four independent

ports, each of which can operate in one of three modes:

manual, semi-auto, or auto. A fourth mode, shutdown,

disables the port (see Table 3).

Table 3. Operating Modes

MODE

AUTO

OPMD

DETECT/

CLASS

POWER-UP

AUTOMATIC

THRESHOLD

ASSIGNMENT

Auto

11b

Enabled

at Reset

Automatically

Yes

11b

Host

Enabled

Automatically

Yes

Semi-auto

10b

Host

Enabled

Upon Request

Manual

01b

Once

Upon

Request

Upon Request

Shutdown

00b

Disabled

In manual mode, the port waits for instructions from the

host system before taking any action. It runs a single

detection, or detection and classification cycle when com-

manded to by the host, and reports the result in its Port

Status register. The host system can command the port

to apply or remove power at any time.

In semi-auto mode, the port repeatedly attempts to detect

and classify any PD attached to it. It reports the status of

these attempts back to the host, and waits for a command

from the host before applying power to the port. The host

must enable detection and classification.

APPLICATIONS INFORMATION

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