LTC4258 datasheet(14/28 Pages) LINER

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LTC4258

4258p

APPLICATIO S I FOR ATIO

The LTC4258 provides a complete solution for detection

and powering of PD devices in an IEEE 802.3af compliant

system. The LTC4258 consists of four independent ports,

each with the ability to detect, classify, and provide iso-

lated –48V power to a PD device connected to it. The

LTC4258 senses removal of a PD with IEEE 802.3af

compliant AC or DC methods and turns off –48V power

when the PD is removed. An internal control circuit takes

care of system configuration and timing, and uses an I2C

interface to communicate with the host system.

OPERATING MODES

Each LTC4258 port can operate in one of four modes:

Manual, Semiauto, Auto or Shutdown. The operating

mode for a port is set by the appropriate bits in the

Operating Mode register. The LTC4258 will power up with

all ports in Shutdown mode if the external AUTO pin is tied

low; if AUTO is high, all ports will wake up in Auto mode.

The operating mode can be changed at any time via the I2C

interface, regardless of the state of the AUTO pin.

• In Manual mode, a port will wait for instructions from

the host system before taking any action. It will run

single detection or classification cycles when com-

manded, and will report results in the Port Status

registers. When the host system decides it is time to

turn on or off power to a port, it can do so by setting

the appropriate Power On/Off bits in the Power Enable

PB register regardless of the current status of detec-

tion or classification.

• In Semiauto mode, the port will repeatedly attempt to

detect and classify a PD device attached to the link. It

will report this information in its Port Status register,

and wait for the host system to set the appropriate

Power On bit in the Power Enable PB register before

applying power to the port.

• In Auto mode, the port will detect and classify a PD

device connected to it, then immediately turn on the

power if detection was successful regardless of the

result of classification.

• In Shutdown mode, the port is disabled and will not detect

or power a PD. Also, the detect and fault event bits, status

bits and enable bits for the port are reset to zero.

Regardless of which mode it is in, the LTC4258 will

remove power automatically from any port that generates

a tSTART or tICUT overcurrent fault event (see tICUT Timing

and tSTART Timing sections). It will also automatically

remove power from any port that generates a disconnect

event if the appropriate Disconnect Enable bit is set in the

Disconnect Enable register. The host controller may also

remove power at any time by setting the appropriate

Power Off bit in the Power Enable PB register.

Power-On RESET

At turn-on or any time the LTC4258 is reset (either by

pulling the RESET pin low or writing to the global Reset All

bit), all the ports turn off and all internal registers go to a

predefined state, shown in Table 1.

Several of the registers assume different states based on

the state of the AUTO pin at reset. The default states with

AUTO high allow the LTC4258 to detect and power up a PD

in Automatic mode, even if nothing is connected to the I2C

interface.

SIGNATURE DETECTION

The IEEE defines a specific pair-to-pair PD signature

resistance that identifies a device that can accept

Power over Ethernet in accordance with the 802.3af

specification. When the port voltage is below 10V, an

802.3af compliant PD will have a 25k signature resistance.

Figure 11 illustrates the relationship between the PD

signature resistance (white box from 23.75k to 26.25k)

and required resistance ranges the PSE must accept

(white box) and reject (gray boxes). According to the

802.3af specification, the PSE may or may not accept

resistances in the two ranges of 15k to 19k and 26.5k to

33k. Note that the black box in Figure 11 represents the

150

Ω pair-to-pair termination used in legacy 802.3 de-

vices like a computer’s network interface card (NIC) that

cannot accept power.

Figure 11. IEEE 802.3af Signature Resistance Ranges

RESISTANCE

PSE

Ω

10k

15k

4258 F11

19k

26.5k

26.25k

23.75k

150

Ω (NIC)

20k

30k

33k

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