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

MP8007 Rev.1.0

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22

6/27/2016

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GND

FB1

Na

R1

R2

Figure 10: Feedback in isolation application

When the primary-side power MOSFET turns

off, the auxiliary-winding voltage is sampled.

The output voltage is estimated:

S

REF1

1

2

OUT

D1F

2A

N

V(R

R )

VV

RN







(12)

Where,

NS is the transformer secondary-side winding

turns.

NA is the transformer auxiliary winding turns.

VD1F is the rectifier diode forward drop.

VREF1 is the reference voltage of FB1 (1.99V,

typically).

When the primary-side power MOSFET turns

on, the auxiliary winding forces a negative

voltage to FB1. The FB1 voltage is clamped to

less than -0.7V internally, but the clamp current

should be limited to less than -0.5mA by R1.

For example, if the auxiliary winding forces

-11V to R1, to make the current flowing from

FB1 to R1 lower than -0.5mA, R1 resistance

must be higher than 22kΩ (if ignoring R2

current).

Generally, select R2 with a 10kΩ to 50kΩ

resistor

to

limit

noise

and

provide

an

appropriate R1 for the -0.5mA negative current

limit.

In buck application, the feedback pin is FB2.

The output voltage can be estimated:

12

OUT

REF2

2

RR

VV

R







(13)

Where, VREF2 is the reference voltage of FB2

-1.88V, typically.

Maximum Switching Frequency

When DCDC converter works in DCM, the

frequency reaches its maximum value during a

full-load condition. The maximum frequency is

affected by the peak current limit, the

inductance, and the input/output voltage.

Generally, design the maximum frequency must

be lower than 200kHz.

In buck mode, the maximum frequency occurs

when the buck runs in critical continuous

conduction mode. The frequency can be

calculated:

IN

OUT

OUT

SW _ MAX

LIM

IN

(V

V

) V

F

IL V







(14)

Where, ILIM is the IPK set by the current limit

resistor.

With a lighter load, the frequency is lower than

the maximum frequency above.

In

flyback

mode,

design

the

maximum

frequency with the minimum input voltage and

the maximum load condition. Calculate the

frequency with Equation:

SW

ON

CON

DELAY

1

F

TT

T





(15)

Where:

TON is the MOSFET one pulse turn-on time

determined with Equation:

LIM

M

ON

IN

IL

T

V





(16)

LM

is

the

transformer

primary-winding

inductance.

TCON is the rectifier diode current conducting

time and can be calculated:

SLIM

M

CON

POUT

D1F

NI

L

T

N(V

V )







(17)

Where, NS is the transformer secondary-side

winding turns. NP is the transformer primary-

side winding turns.

TDELAY is the resonant delay time from the

rectifier diode current drop to 0A to the

auxiliary-winding voltage drop to 0V. The

resonant time can be tested on the board

(estimate around 0.5μs).

In flyback mode, the DCDC converter samples

the feedback signal within 3μs after the primary-