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

MP8007 Rev.1.0

www.MonolithicPower.com

21

6/27/2016

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In PG high condition, PG pin is pulled up by

internal 30μA current source while clamped by

one 5.5V zener between PG and RTN. In PG

low condition, internal 30μA current source is

disabled and PG pin is pulled low by about

460k Ω pull-down resistor between PG and

RTN(GND). Generally, float PG for automatic

startup after power is connected. PG pin can be

pulled low externally but the signal sink current

capability must be higher than the internal

current source. The zener on PG pin is used to

clamp internal 30μA current, do not connect

external signal with higher than 5.5V voltage to

PG pin.

T2P Indicator Connection

The T2P pin is an active-low, open-drain output

which indicates the presence of a Type-2 PSE

or AUX is enabled. An opto-coupler is usually

used as the interface from the T2P pin to

circuitry on output of the converter as figure 8

shown. A high-gain opto-coupler and a high-

impedance (for example, CMOS) receiver are

recommended.

Figure 8: T2P Indicator Circuit

Considering T2P sinking current (2mA typical),

T2P output low voltage 0.1V and diode forward

voltage drop, choose RT2P=23.7kΩ to match the

typical 48V VDD input. Suppose VOUT of DCDC

converter is 12V, usually choose RT2P-O=20kΩ

based on the CRT even it may vary with

temperature, LED bias current and aging.

If lighten a LED from VDD to T2P to indicate the

T2P`s activity, the RT2P’s resistance can be

higher to match the LED`s max current and

reduce the power-loss.

VCC Power Supply Setting

The VCC voltage is charged through the internal

LDO by VDD. Normally, VCC is regulated at

5.4V, typically. A capacitor no less than 1µF is

recommended for decoupling between VCC and

GND.

In flyback mode, VCC can be powered from the

transformer auxiliary winding to save the high-

voltage LDO power loss.

Figure 9: Supply VCC from auxiliary winding

The auxiliary winding supply voltage can be

calculated with Equation (11):

A

CC

OUT

D1F

DAUXF

S

N

V(V

V ) V

N







(11)

Where NA and NS are the turns of the auxiliary

winding and the output winding, VD1F is the

output rectifier diode voltage drop, and VDAUXF is

the DAUX voltage drop in Figure 9.

VCC voltage is clamped at about 6.2V by one

internal Zener diode. The clamp current

capability is about 1.2mA. If the auxiliary

winding power voltage is higher than 6.2V

(especially in a heavy-load condition), a series

resistor (RAUX) is necessary to limit the current

to VCC. For simple application, supply the VCC

power through the internal LDO directly.

Converter Output Voltage Setting

In DCDC converter, there are two feedback

pins for different application modes.

In flyback mode, the converter detects the

auxiliary winding voltage from FB1. R1 and R2

are the resistor dividers for the feedback

sampling

(see

Figure

10).