NCP1081

http://onsemi.com

4

SIMPLIFIED APPLICATION DIAGRAMS

Figure 4. Non−Isolated Fly−back with Extra Winding

Cpd

NCP1081

GATE

RTN

FB

VPORTN2

VDDH

VDDL

Rcs

Cvddh

C1comp

C2comp

Rcomp

Css

M1

R4

VPORTN1

CLASS

ARTN

ILIM1

INRUSH

TEST1

TEST2

Rclass

Rilim1

Rinrush

R1

R2

UVLO

CS

VPORTP

Rslope

D1

D2

T1

R5

Voutput

Spare

Pairs

RJ−45

DB1

DB2

LD1

Rd1

Cvddl

nCLASS_AT

R3

Cload

Rosc

Data

Pairs

Figure 4 shows the same non−isolated fly−back configuration as Figure 3, but adds a 12 V auxiliary bias winding on the

transformer to provide power to the NCP1081 DC−DC controller via its VDDH pin. This topology shuts off the current flowing

from VPORTP to VDDH and therefore reduces the internal power dissipation of the PD, resulting in higher overall power

efficiency.

Figure 5. Non−Isolated Forward Converter

Cpd

NCP1081

GATE

RTN

FB

VPORTN2

VDDH

VDDL

Rcs

Cvddh

C1comp

C2comp

Rcomp

Css

M1

Cload

R3

R4

VPORTN1

CLASS

ARTN

ILIM1

INRUSH

TEST1

TEST2

Rclass

Rilim1

Rinrush

R1

R2

UVLO

CS

VPORTP

L1

Rslope

Voutput

T1

D1

D2

D3

Spare

Pairs

RJ−45

DB1

DB2

LD1

Rd1

Cvddl

nCLASS_AT

Data

Pairs

Rosc

Figure 5 shows the NCP1081 used in a non−isolated forward topology.

High Power Considerations

The NCP1081 is designed to implement various

configurations of high−power PoE systems including those

based on the IEEE802.3at standard. High power operation

can be enabled by a Dual Event Layer 1 classification or a

Single Event Layer 1 classification combined with a Layer 2

high power classification. The NCP1081 also supports

proprietary designs capable of delivering 25 W to 40 W to

the load in two−pair configurations. A separate application

note describes these implementations (AND8332).