ACT4065

APPLICATION INFORMATION

OUTPUT VOLTAGE SETTING

Figure 3. Output Voltage Setting

Figure 3 shows the connections for setting

the output voltage. Select the proper ratio of the









−

=

1

V

293

.

1

V

R

R

OUT

2

FB

1

FB

(1)

INDUCTOR SELECTION

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:

RIPPLE

OUTMAX

SW

IN

OUT

IN

OUT

K

I

f

V

)

V

V

(

V

L

−

•

=

(2)

correspond to the peak-to-peak ripple current

being 30% of the maximum output current.

With this inductor value (Table 1), the peak

sure that this peak inductor current is less that

the 3A current limit. Finally, select the inductor

core size so that it does not saturate at 3A.

Table 1. Typical Inductor Values

1.5V

1.8V

2.5V

3.3V

5V

L

10μH

10μH 15μH 22μH 33μH

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

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:

ESR

RIPPLE

OUTMAX

RIPPLE

R

K

I

V

=

OUT

2

SW

IN

LC

f

28

V

•

+

(3)

is the output capacitance. In the case of ceramic

not contribute to the ripple. Therefore, a lower

capacitance value can be used for ceramic type.

In the case of tantalum or electrolytic type, the

ripple current. In that case, the output capacitor

is chosen to have sufficiently low ESR.

For ceramic output type, typically choose a

capacitance 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 current when the High-Side Power

Switch is off. The Schottky diode must have

current rating higher than the maximum output

current and the reverse voltage rating higher

than the maximum input voltage.

Active-Semi, Inc.

- 5 -

www.active-semi.com

ACT4065

FB