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

MP8007 Rev.1.0

www.MonolithicPower.com

20

6/27/2016

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APPLICATION INFORMATION

Detection Resistor

In the Detection Mode, a resistor connected

between DET and VDD pin is needed as a load

to the PSE. The resistance is calculated as a

ΔV/ΔI, with an acceptable range of 23.7kΩ to

26.3kΩ. Use a typical value of 24.9kΩ as

detection resistor.

Classification Resistor

In order to distribute power to as many loads as

possible from PSE, a resistor between CLASS

and VSS pins is used to classify the PD power

level, which draws a fixed current set by

classification resistor. The power supplied to

PD set by classification resistor is shown in

Table 1. Typical voltage on CLASS pin is 1.16V

in classification range, and it produces about

33mW power loss on class resistor in Class 3

condition.

Protection TVS

To limit input transient voltage within the

absolute maximum rating, a TVS across the

rectified voltage (VDD-VSS) must be used. A

SMAJ58A, or equivalent, is recommended for

general indoor applications. Outdoor transient

levels or special applications require additional

protection.

PD Input Capacitor

An input bypass capacitor (from VDD to VSS)

of 0.05μF to 0.12μF is needed for IEEE

802.3af/at standard specification. Typically a

0.1μF, 100V ceramic capacitor is used.

Wall Power Adaptor Detection Circuit

When an auxiliary power source such as a wall

power adapter is used to power the device, the

divider resistors RADPUP, RADPDOWN and DADP3

must be chosen as shown in figure 7 to satisfy

the Equation (1) for correct wall power adaptor

detection.

RADPUP with typical 3kΩ value is suggested to

balance the power loss and DADP1&DADP2

leakage current discharge.

VDD

AUX

VSS

RTN

VADP

Adaptor GND

RADPUP

R ADPDOWN

DADP1

DADP2

DADP3

PG

Figure 7: Wall Adaptor Detection Circuit

To prevent the converter from operating at an

excessively low adapter voltage, choose a

startup voltage, VSTART approximately 80% of

nominal. Assuming that the adapter voltage is

48V, Let RADPUP=3kΩ, RADPDOWN=8.06kΩ and

DADP3=30V as Equation (1). Re-check the

adapter turn-on and turn-off voltage:

ADPUP

ADPDOWN

ADP ON

ADPUP

RR

V30 2.3

38.5V

R











(9)

ADPUP

ADPDOWN

ADP OFF

ADPUP

RR

V

30 0.6

32.2V

R











(10)

The VDD-AUX voltage differential voltage is

4.88V when adapter input is 48V. If much

higher adapter voltage is applied and divided

voltage on AUX pin is higher than 6.5V,

RADPDOWN and DADP3 must be able to limit the

current from AUX to adapter-GND less than

3mA, or else additional resistor from tap of

resistor divider to AUX pin is needed to limit the

current.

One small package Schottky diode with 100V

voltage rating (such as BAT46W) is usually

suggested for DADP1. The voltage rating of DADP2

must also be 100V or higher while current rating

must be higher than load current. Low voltage

drop Schottky diode (such as SS1H10) is

recommended to reduce conduction power-loss.

Power Good (PG) Indicator Signal

MP8007 integrates one PG indicator. PG pin is

used to indicate the PD inrush period finishes

and enable the DCDC converter internally. The

PG pin is an active-high output with internal

driven, consequently it can be floated to enable

DCDC converter. Pull PG pin low externally can

disable the DCDC regulator of MP8007.