LTC4258 datasheet(16/28 Pages) LINER

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LTC4258

4258p

measures the port voltage through the DETECT

n pin. Note

that class 4 is presently specified by the IEEE as reserved

for future use. Figure 13 shows a PD load line, starting with

the shallow slope of the 25k signature resistor below 10V,

then drawing the classification current (in this case, class 3)

between 14.5V and 20.5V. The LTC4258’s load line for clas-

sification is also shown in Figure 13. It has low impedance

until current limit at 65mA (typ).

The LTC4258 will classify a port immediately after a

successful detection cycle in Semiauto or Auto modes, or

when commanded to in Manual mode. It measures the PD

classification signature current by applying 18V (typ) to

the port and measuring the resulting current. It reports the

detected class in the Class Status bits in the correspond-

ing Port Status register. Note that in Auto mode, the port

will power up regardless of which class is detected.

The classification circuitry is disabled when the port is in

Shutdown mode, powered up, or the corresponding Class

Enable bit is cleared.

Gate Currents

Once the decision has been made to turn on power to a

port, the LTC4258 uses a 50

µA current source to pull up

on the GATE pin. Under normal power-up circumstances,

the MOSFET gate will charge up rapidly to VT (the MOSFET

threshold voltage), the MOSFET current will rise quickly to

the current limit level and the GATE pin will be servoed to

maintain the proper IINRUSH charging current. When out-

put charging is complete, the MOSFET current will fall and

the GATE pin will be allowed to continue rising to fully

enhance the MOSFET and minimize its on resistance. The

final VGS is nominally 13V. When a port is turned off, a

µA current source pulls down on the GATE pin, turning

the MOSFET off in a controlled manner.

No External Capacitors

No external capacitors are required on the GATE pins for

active current limit stability, lowering part count and cost.

This also allows the fastest possible turn-off under severe

overcurrent conditions, providing maximum safety and

protection for the MOSFETs, load devices and board traces.

Connecting capacitors to the external MOSFET gates can

adversely affect the LTC4258’s ability to respond to a

shorted port.

Inrush Control

The 802.3af standard lists two separate maximum current

limits, ILIM and IINRUSH. Because they have identical val-

ues, the LTC4258 implements both as a single current

limit using VLIM (described below). Their functions are

differentiated through the use of tICUT and tSTART, respec-

tively (see tICUT Timing and tSTART Timing sections). To

maintain consistency with the standard, the IINRUSH term

is used when referring to an initial tSTART power-up event.

When the LTC4258 turns on a port, it turns on the MOSFET

by pulling up on the gate. The LTC4258 is designed to

power up the port in current limit, limiting the inrush

current to IINRUSH.

The port voltage will quickly rise to the point where the PD

reaches its input turn-on threshold and begins to draw

current to charge its bypass capacitance, slowing the rate

of port voltage increase.

APPLICATIO S I FOR ATIO

Figure 13. PD Classification

VOLTAGE (VCLASS)

510

4258 F13

TYPICAL

CLASS 3

PD LOAD

LINE

48mA

33mA

PSE LOAD LINE

23mA

14.5mA

6.5mA

CLASS 4

CLASS 2

CLASS 1

CLASS 0

CLASS 3

OVER

CURRENT

POWER CONTROL

The primary function of the LTC4258 is to control the

delivery of power to the PSE port. It does this by control-

ling the gate drive voltage of an external power MOSFET

while monitoring the current via a sense resistor and the

output voltage at the OUT pin. This circuitry serves to

couple the raw isolated –48V input supply to the port in a

controlled manner that satisfies the PD’s power needs

while minimizing disturbances on the –48V backplane.

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