L3380

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

7 of 9

www.unisonic.com.tw

QW-R502-099,A

APPLICATION CIRCUIT INFORMATION (Cont.)

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

COFF

QI

T

∆ =•

we can write

1

()

LO

d

QI

I

f

−

∆=

−

•

6. Output ripple voltage is given by

()

PPC

L

O

VU

ESR

I

I

=∆

+

•

−

(ESR: equivalent series resistance of the output capacitor)

()

PPL

O

Q

VESR

I

I

C

∆

=+

•

−

Then we give the following example about choosing external components by considering the design parameters.

Design parameters:

IN

O

PP

Assume

D

S

2.1 0.3 1.5

0.429

2.1 0.3 0.3

OD

IN

OD

S

UU

U

d

UU

U

+−

+−

==

=

+−

+

−

In order to generate the desired output current and ICR, the value of inductor should meets the following formula

L ≤

(1– d)

ICR

=

(1– 0.429)

0.2×0.2A×300000HZ

= 24.5uH

Calculate the average current and the peak current of inductor

0.2

0.35

1

1 0.429

O

L

I

A

I

A

d

==

=

−−

11

(1

)

0.35

(1

0.2)

0.385

22

PK

L

I

IICR

A

A

=+

=

× + ×

=

So, we make a trial of choosing a 22uH inductor that allowable maximum current is lager than 0.385A.

Determine the delta charge stores in C3 during charging up

1

1 0.429

(

)

(0.35

0.2 )

0.286

300000

LO

d

QI

I

A

A

uC

fHZ

−−

∆=

−

•

=

−

×

=

Assume the ESR of C3 is 0.15Ω, determine the value of C3

6

0.286 10

3.69

(

)

0.1 0.15

(0.35

0.2 )

PP

L

O

QC

CuF

VESR

I

I

A

A

−

∆×

≥=

=

−•

−

−

Ω×

−

Therefore, a Tantalum capacitor with value of 10uF and ESR of 0.15Ω can be used as output capacitor. However,

the optimized value should be obtained by experiment.