Active-Semi

ACT4050

Rev 2, 01-Jul-11

- 6 -

www.active-semi.com

Copyright © 2011 Active-Semi, Inc.

Innovative Power

TM

Figure 1 shows the connections for setting the

output voltage. Select the proper ratio of the two

The inductor maintains a continuous current to the

output load. This inductor current has a ripple that is

dependent on the inductance value: higher

inductance reduces the peak-to-peak ripple current.

The trade off for high inductance value is the

increase in inductor core size and series resistance,

and the reduction in current handling capability. In

general, select an inductance value L based on

ripple current requirement:

Inductor Selection

APPLICATIONS INFORMATION

correspond to the peak-to-peak ripple current being

30% of the maximum output current.

With a selected inductor value the peak-to-peak

inductor current is estimated as:

The peak inductor current is estimated as:

The maximum output current is calculated as:

(1)

(2)

Figure 1:

Output Voltage Setting

⎟

⎠

⎞

⎜

⎝

⎛

−

=

1

V

817

.

0

V

R

R

OUT

2

FB

1

FB

()

RIPPLE

OUTMAX

SW

IN

OUT

IN

OUT

K

I

f

V

V

V

V

L

−

×

=

(6)

ESR

RIPPLE

OUTMAX

RIPPLE

R

K

I

V

=

OUT

2

SW

IN

LC

f

28

V

×

+

Electrical Characteristics Table.

Input Capacitor

The input capacitor needs to be carefully selected to

maintain sufficiently low ripple at the supply input of

the converter. A low ESR capacitor is highly

recommended. Since large current flows in and out

of this capacitor during switching, its ESR also

affects efficiency.

The input capacitance needs to be higher than

10µF. The best choice is the ceramic type,

however, low ESR tantalum or electrolytic types

may also be used provided that the RMS ripple

current rating is higher than 50% of the output

current. The input capacitor should be placed close

to the IN and G pins of the IC, with the shortest

traces possible. In the case of tantalum or

electrolytic types, they can be further away if a small

parallel 0.1µF ceramic capacitor is placed right next

to the IC.

Output Capacitor

The output capacitor also needs to have low ESR to

keep low output voltage ripple. The output ripple

voltage is:

small and does not contribute to the ripple.

Therefore, a lower capacitance value can be used for

ceramic type. In the case of tantalum or electrolytic

by the ripple current. In that case, the output

capacitor is chosen to have sufficiently low ESR.

For ceramic output capacitor, typically choose a

capacitance of about 22µF. For tantalum or

electrolytic capacitors, choose a capacitor with less

than 50mΩ ESR.

Rectifier Diode

Use a Schottky diode as the rectifier to conduct

current when the High-Side Power Switch is off. The

Schottky diode must have current rating higher than

the maximum output current and a reverse voltage

rating higher than the maximum input voltage.

Output Voltage Setting

ACT4050

FB

Note: To achieve best performance with 12V input application,

we recommend to use output voltage greater than 1.4V.

(

)

SW

IN

OUT

IN

OUT

PK

LPK

f

V

L

V

V

V

I

×

×

×

=

-

-

(3)

PK

LPK

LOADMAX

LPK

I

2

1

I

I

-

+

=

(4)

PK

LPK

LIM

OUTMAX

I

2

1

I

I

-

-

=

(5)