LTC4268IDKD-1#PBF【PDF 21/46 页】IC PD HIGH POWER W/CNTRL 32-DFN

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LTC4268-1

PD is required to accept power in either polarity on both

the data and spare inputs; therefore it is common to install

diode bridges on both inputs in order to accommodate the

different wiring configurations. Figure 9 demonstrates an

implementation of the diode bridges to minimize heating.

The IEEE 802.3af specification also mandates that the

leakage back through the unused bridge be less than 28礎

when the PD is powered with 57V.

The LTC4268-1 has several different modes of operation

based on the voltage present between the V

PORTN

and

PORTP

pins. The forward voltage drop of the input diodes

in a PD design subtracts from the input voltage and will

affect the transition point between modes.

The input diode bridge of a PD can consume over 4% of

the available power in some applications. Schottky diodes

can be used in order to reduce power loss. The LTC4268-1

is designed to work with both standard and Schottky

diode bridges while maintaining proper threshold points

for IEEE 802.3af compliance.

Input Capacitor

The IEEE 802.3af/at standard includes an impedance

requirement in order to implement the AC disconnect

function. A 0.1礔 capacitor (C14 in Figure 9) is used to

meet the AC impedance requirement.

Input

Series Resistance

Linear Technology has seen the customer community cable

discharge requirements increase by nearly 500,000 times

the original test levels. The PD must survive and operate

reliably not only when an initially charged cable connects

and dissipates the energy through the PD front end, but

also when the electrical power system grounds are subject

to very high energy events (e.g., lightning strikes).

In these high energy events, adding 10?series resistance

into the V

PORTP

pin greatly improves the robustness of

the LTC4268-1 based PD (see Figure 9). The TVS limits

the voltage across the port while the 10?and 0.1礔 ca-

pacitance reduces the edge rate the LT4268-1 encounters

across its pin. The added 10?series resistance does not

operationally affect the LTC4268-1 PD Interface, nor does

it affect its compliance with the IEEE 802.3 standard.

Transient Voltage Suppressor

The LTC4268-1 specifies and absolute maximum volt-

age of 100V and is designed to tolerate brief overvoltage

events. However, the pins that interface to the outside

world can routinely see excessive peak voltages. To pro-

tect the LTC4268-1, install a transient voltage suppressor

(D3) between the input diode bridge and the LTC4268-1,

as shown in Figure 9. An SMAJ58A is recommended for

typical PD applications. However, an SMBJ58A may be

preferred in applications where the PD front end must

absorb higher energy discharge events.

Auxiliary Power Source

In some applications, it may be necessary to power the

PD from an auxiliary power source such as a wall adapter.

The auxiliary power can be injected into the PD at several

locations and various trade-offs exist. Figure 10 demon-

strates four methods of connecting external power to a PD.

Option 1 in Figure 10 inserts power before the LTC4268-1

interface controller. In this configuration, it is necessary

for the wall adapter to exceed the LTC4268-1 UVLO turn-

on requirement. This option provides input current limit

for the adapter, provides a valid power good signal and

simplifies power priority issues. As long as the adapter

applies power to the PD before the PSE, it will take priority

and the PSE will not power up the PD because the external

power source will corrupt the 25k signature. If the PSE

is already powering the PD, the adapter power will be in

parallel with the PSE. In this case, priority will be given to

the higher supply voltage. If the adapter voltage is higher,

the PSE may

remove the port voltage since no current will

be drawn from the PSE. On the other hand, if the adapter

voltage is lower, the PSE will continue to supply power to

the PD and the adapter will not be used. Proper operation

will occur in either scenario.

Option 2 applies power directly to the DC/DC converter.

In this configuration the adapter voltage does not need

to exceed the LTC4268-1 turn-on UVLO requirement and

can be selected based solely on the PD load requirements.

It is necessary to include diode D9 to prevent the adapter

from applying power to the LTC4268-1. Power priority

issues require more intervention. If the adapter voltage

is below the PSE voltage, then the priority will be given

applicaTions inForMaTion

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