UC3380

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 9

www.unisonic.com.tw

QW-R502-099.D

APPLICATION CIRCUIT INFORMATION

The following equations show the relation of the basic design parameters.

1. Refer to the application circuit, the increasing inductor current when the switch is turn on is given by the following

equation

11

()

LL ON

IN

S

d

iU T

U

U

LL

f

+

Δ=

=

−

(

IN

U : input voltage:

S

U : transistor saturation voltage)

The decreasing inductor current when the switch is turn off can derive by the equation below

11

1

()

LL OFF

O

D

IN

d

iU T

U

U

U

LL

f

−

−

Δ=

= −

+

−

(

D

U ：diode forward voltage)

according to

0

LL

ii

+−

Δ+ Δ

= , the duty ratio is given by

OD

IN

OD

S

UU

U

d

UU

U

+−

=

+−

2. The average current flowing through the inductor is

1

O

L

I

I

d

=

−

3. We note that

(1

)

OL

Id I

=−

then we can write:

(1

OL

L

I

Id

i

i

= −• Δ

Δ

substituting

1

LL OFF

iU T

L

Δ=

L

i

Δ

for equation above, output current is given by

11

(1

)

OL OFF

Id

U T

ICR L

=−

•

•

( ICR =

L

L

i

I

Δ

)

11

1

(1

)

(

)

OD

IN

d

Id

Uo U

U

ICR L

f

−

=−

•

•

+

−

2

(1

)

O

DIN

O

UU

U

Id

ICR L f

+−

=−

•

••

derive that

4. The peak current of the inductor is given by

1

2

PK

L

L

II

i

=+ Δ

1

2

L

PK

L

L

L

i

II

I

I

Δ

=+

•

according to ICR =

L

L

i

I

Δ

derive that

1

2

PK

L

L

II

ICR I

=+

•

Then derive the following equation for peak current of inductor

1

(1

)

2

PK

L

II

ICR

=+

5. Charge stores in C3 during charging up is given by

COFF

QI

T

Δ= •

we can write

1

()

LO

d

QI

I

f

−

Δ=

−

•