MP8007 ― IEEE 802.3AF PD WITH 13W FLYBACK / BUCK CONVERTER

MP8007 Rev.1.0

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25

6/27/2016

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5. Keep the feedback trace far away from

noise sources (such as SW). The trace

connecting FB1 should be short.

6. Use a single point connection between

power GND and signal GND. Vias around

GND

and

the

thermal

pad

are

recommended to lower the die temperature.

Refer to Figure 12 for flyback circuit layout,

which is referred to schematic

on page 1.

Figure 12: Recommended flyback layout

For buck circuit:

1. Keep the input loop as short as possible

between the input capacitor, rectifier diode,

SW, and GND plane for minimal noise and

ringing.

2. Keep the output loop between the rectifier

diode, the output capacitor, and the inductor

as short as possible.

3. Place the VCC capacitor close to VCC for

the best decoupling. The current setting

resistor R3 should be placed as close to

ILIM and AGND as possible.

4. Connect the output voltage sense and VDD

power supply from the output capacitor with

parallel traces. The feedback trace should

be far away from noise sources (such as

SW). The trace connected to FB2 should be

short. The trace for VDD power should be

wider.

5. Use a single point connection between

power GND and signal GND. Vias around

GND

and

the

thermal

pad

are

recommended to lower the die temperature

Refer to Figure 13 for buck circuit layout.

Via

Top Layer

Bottom Layer

R7

D3

R8

R5 R2 R1

C3

R6

D1

C2

L1

U1

R3

C5

C1

D5

D4

Figure 13: Recommended buck layout

Design Example

Below is a design example following the

application

guidelines

for

the

following

specifications:

Table 2 – Flyback Design Example

VDD-VSS

37V–57V (PoE Supply)

RDET

24.9kΩ

RCLASS

41.2Ω

VADAPTER

48V

VOUT

12V

IOUT

1A

The typical application circuit in Figure 14

shows the detailed application schematic, and

is the basis for the typical performance

waveforms. Typically, the device is powered by

PSE (VDD-VSS=48V). When an adapter voltage

above than 38.5V presents, the internal

MOSFET between RTN and VSS turns off,

instead the device is powered by the adapter

whatever the PSE voltage is. For more detailed

device applications, please refer to the related

Evaluation Board Datasheets.