LTC4258 datasheet(20/42 Pages) LINER

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LTC4278

4278fc

Classification Resistor (RCLASS)

The RCLASS resistor sets the classification load current,

corresponding to the PD power classification. Select the

value of RCLASS from Table 2 and connect the resistor

between the RCLASS and VPORTN pins as shown in Figure

4, or float the RCLASS pin if the classification load cur-

rent is not required. The resistor tolerance must be 1%

or better to avoid degrading the overall accuracy of the

classification circuit.

Load Capacitor

The IEEE 802.3af/at specification requires that the PD

maintains a minimum load capacitance of 5μF and does

not specify a maximum load capacitor. However, if the

load capacitor is too large, there may be a problem with

inadvertent power shutdown by the PSE.

This occurs when the PSE voltage drops quickly. The input

diode bridge reverses bias, and the PD load momentarily

powers off the load capacitor. If the PD does not draw

power within the PSE’s 300ms disconnection delay, the

PSE may remove power from the PD. Thus, it is necessary

to evaluate the load current and capacitance to ensure that

an inadvertent shutdown cannot occur.

The load capacitor can store significant energy when fully

charged. The PD design must ensure that this energy is

not inadvertently dissipated in the LTC4278. For example,

if the VPORTP pin shorts to VPORTN while the capacitor

is charged, current will flow through the parasitic body

diode of the internal MOSFET and may cause permanent

damage to the LTC4278.

T2P Interface

When a 2-event classification sequence successfully

completes, the LTC4278 recognizes this sequence, and

provides an indicator bit, declaring the presence of a

Type 2 PSE. The open-drain output provides the option

to use this signal to communicate to the LTC4278 load,

or to leave the pin unconnected.

Figure 8 shows two interface options using the T2P pin

and the opto-isolator. The T2P pin is active low and con-

nects to an opto-isolator to communicate across the DC/

DC converter isolation barrier. The pull-up resistor RP is

sized according to the requirements of the opto-isolator

operating current, the pull-down capability of the T2P pin,

and the choice of V+. V+ for example can come from the

PoE supply rail (which the LTC4278 VPORTP is tied to), or

from the voltage source that supplies power to the DC/

DC converter. Option 1 has the advantage of not drawing

power unless T2P is declared active.

Shutdown Interface

To corrupt the signature resistance, the SHDN pin can be

driven high with respect to VPORTN. If unused, connect

SHDN directly to VPORTN.

Auxiliary Power Source

In some applications, it is desirable to power the PD from

an auxiliary power source such as a wall adapter.

Auxiliary power can be injected into an LTC4278-based

PD at the input of the LTC4278 VPORTN, at VNEG, or even

the power supply output. In addition, some PD applications

maydesireauxiliarysupplydominanceormaybeconfigured

APPLICATIONS INFORMATION

4278 F08

OPTION 1: SERIES CONFIGURATION FOR ACTIVE LOW/LOW IMPEDANCE OUTPUT

–54V

PSE

TO PD LOAD

VPORTP

LTC4278

VPORTN

T2P

V+

OPTION 2: SHUNT CONFIGURATION FOR ACTIVE HIGH/OPEN COLLECTOR OUTPUT

–54V

PSE

TO PD LOAD

VPORTP

LTC4278

VPORTN

VNEG

T2P

V+

Figure 8. T2P Interface Examples

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