ACT4012A

Rev0, 16-May-08

Innovative Power

TM

- 5 -

www.active-semi.com

Copyright © 2008 Active-Semi, Inc.

Figure 1 shows the connections for setting the out-

put voltage. Select the proper ratio of the two feed-

The inductor maintains a continuous current to the

output load. This inductor current has a ripple that is

dependent on the inductance value higher induc-

tance 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:

1.5V

1.8V

2.5V

3.3V

5V

L

7.5µH

10µH

12µH

15µH

22µH

tance. In the case of ceramic output capacitors,

ripple. Therefore, a lower capacitance value can be

used for ceramic type. In the case of tantalum or

multiplied by the ripple current. In that case, the

output capacitor is chosen to have sufficiently low

ESR.

For ceramic output type, typically choose a capaci-

tance of about 22µF. For tantalum or electrolytic

type, choose a capacitor with less than 50mΩ ESR.

Rectifier Diode

Use a Schottky diode as the rectifier to conduct cur-

rent 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.

Inductor Selection

APPLICATIONS INFORMATION

spond to the peak-to-peak ripple current being 30%

of the maximum output current.

With this inductor value, the peak inductor current is

ductor current is less that the 3A current limit. Fi-

nally, select the inductor core size so that it does

not saturate at 3A.

(1)

(2)

Figure 1:

Output Voltage Setting

⎟

⎠

⎞

⎜

⎝

⎛

−

=

1

V

293

.

1

V

R

R

OUT

2

FB

1

FB

()

RIPPLE

OUTMAX

SW

IN

OUT

IN

OUT

K

I

f

V

V

V

V

L

−

×

=

(3)

ESR

RIPPLE

OUTMAX

RIPPLE

R

K

I

V

=

OUT

2

SW

IN

LC

f

32

V

×

+

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 rec-

ommended. 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, how-

ever, 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 ce-

ramic 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:

Output Voltage Setting

Table 1:

Typical Inductor Values

ACT4012A

FB