Innovative Power

TM

- 5 -

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

ACT4065A

Rev 0, 23-Apr-12

®

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 recom-

mended. Since a 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 possible traces. In the

case of tantalum or electrolytic types, they can be

further away if a small parallel 0.1µF ceramic ca-

pacitor 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:

not contribute to the ripple. Therefore, a lower ca-

pacitance value can be used for ceramic type. In the

case of tantalum or electrolytic type, the ripple is

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 Schotky 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 the reverse volt-

age rating higher than the maximum input voltage.

APPLICATIONS INFORMATION

Output Voltage Setting

Figure 1:

Output Voltage Setting

Figure 1 shows the connections for setting the out-

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

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 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:

factor.

Typically,

choose

correspond to the peak-to-peak ripple current being

30% of the maximum output current.

With this inductor value (Table 1), the peak inductor

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

(1)

⎟

⎠

⎞

⎜

⎝

⎛

−

=

1

V

808

.

0

V

R

R

OUT

2

FB

1

FB

(2)

()

RIPPLE

OUTMAX

SW

IN

OUT

IN

OUT

K

I

f

V

V

V

V

L

−

×

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

+

=

OUT

2

SW

IN

ESR

RIPPLE

OUTMAX

RIPPLE

LC

f

28

V

R

K

I

V

(3)