LTC4257CS8 datasheet(13/20 Pages) LINER

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LTC4257

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APPLICATIO S I FOR ATIO

However, if the standard diode bridge is replaced with a

Schottky bridge, the transition points between modes will

be affected. The application circuit (Figure 11) shows a

technique for using Schottky diodes while maintaining

proper threshold points to meet IEEE 802.3af compliance.

Auxiliary Power Source

In some applications, it may be desirable to power the PD

from an auxiliary power source such as a wall transformer.

The auxiliary power can be injected into the PD at several

locations and various trade-offs exist. Power can be

injected at the 3.3V or 5V output of the isolated power

supply with the use of a diode ORing circuit. This method

accesses the internal circuits of the PD after the isolation

barrier and therefore meets the 802.3af isolation safety

requirements for the wall transformer jack on the PD.

Power can also be injected into the PD interface portion of

the LT4257. In this case, it is necessary to ensure the user

cannot access the terminals of the wall transformer jack

on the PD since this would compromise the 802.3af

isolation safety requirements. Figure 9 demonstrates three

methods of diode ORing external power into a PD. Option

1 inserts power before the LTC4257 while options 2 and 3

insert power after the LTC4257.

If power is inserted before the LTC4257 (option 1), it is

necessary for the wall transformer to exceed the LTC4257

UVLO turn-on requirement and limit the maximum voltage

to 57V. This option provides input current limiting for the

transformer, provides valid power good signaling and sim-

plifies power priority issues. As long as the wall transformer

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

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

will corrupt the 25k signature. If the PSE is already pow-

ering the PD, the wall transformer power will be in parallel

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

supply voltage. If the wall transformer voltage is higher, the

PSE should remove line voltage since no current will be

drawn from the PSE. On the other hand, if the wall trans-

former voltage is lower, the PSE will continue to supply

power to the PD and the wall transformer power will not be

used. Proper operation should occur in either scenario.

Auxiliary power can be applied after the LTC4257 as shown

in option 2. In this configuration, the wall transformer does

not need to exceed the LTC4257 turn-on UVLO requirement;

however, it is necessary to include diode D9 to prevent the

transformer from applying power to the LTC4257. The

transformer voltage requirements will be governed by the

needs of the PD switcher and may exceed 57V. However,

power priority issues require more intervention. If the wall

transformer voltage is below the PSE voltage, then priority

will be given to the PSE power. The PD will draw power from

the PSE while the transformer will sit unused. This configu-

ration is not a problem in a PoE system. On the other hand,

if the wall transformer voltage is higher than the PSE volt-

age, the PD will draw power from the transformer. In this

situation, it is necessary to address the issue of power

cycling that may occur if a PSE is present. The PSE will detect

the PD and apply power. If the PD is being powered by the

wall transformer, then the PD will not meet the minimum

load requirement and the PSE will subsequently remove

power. The PSE will again detect the PD and power cycling

will start. With a transformer voltage above the PSE volt-

age, it is necessary to install a minimum load on the output

of the LTC4257 to prevent power cycling. Refer to the

LTC4257-1 data sheet for an alternative implementation of

option 2 which uses the Signature Disable feature.

The third option also applies power after the LTC4257, while

omitting diode D9. With the diode omitted, the transformer

voltage is applied to the LTC4257 in addition to the load.

For this reason, it is necessary to ensure that the transformer

maintain the voltage between 44V and 57V to keep the

LTC4257 in its normal operating range. The third option has

the advantage of automatically disabling the 25k signature

when the external voltage exceeds the PSE voltage.

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