MP6004—PRIMARY-SIDE REGULATED FLYBACK/BUCK 80V DCDC CONVERTER

MP6004 Rev. 1.0

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18

4/14/2015

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capability is about 1.2 mA. If the auxiliary

winding power voltage is higher than 6.2 V

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

resistor (R8) is necessary to limit the current to

power through the internal LDO directly.

Output Voltage Setting

In MP6004, 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 6).

GND

FB1

Na

R1

R2

Figure 6—Feedback in isolation application

When the primary-side power MOSFET turns

off, the auxiliary-winding voltage is sampled.

The output voltage is estimated with Equation

(12):

S

REF1

1

2

OUT

D1F

2A

N

V(R

R )

VV

RN

×+

=×

−

(12)

Where,

turns.

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 -0.7 V internally, but the clamp current

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

For example, if the auxiliary winding forces

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

FB1 to R1 lower than -0.5 mA), R1 resistance

must be higher than 22 kΩ (if ignoring R2

current).

Generally, select R2 with a 10 kΩ to 50 kΩ

resistor

to

limit

noise

and

provide

an

appropriate R1 for the -0.5 mA negative current

limit.

In buck application, the feedback pin is FB2.

The output voltage can be estimated with

Equation (13):

12

OUT

REF2

2

RR

VV

R

+

=−

×

(13)

-1.88 V, typically.

Maximum Switching Frequency

When MP6004 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 lower than 200 kHz.

In buck mode, the maximum frequency occurs

when the buck runs in critical conduction mode.

The frequency can be calculated with Equation

(14):

IN

OUT

OUT

SW _ MAX

LIM

IN

(V

V

) V

F

IL V

−×

=

××

(14)

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 (15):

SW

ON

CON

DELAY

1

F

TT

T

≤

++

(15)

Where:

determined with Equation (16):

LIM

M

ON

IN

IL

T

V

×

=

(16)

is

the

transformer

primary-winding

inductance.