- 5 -

www.active-semi.com

Copyright © 2006 Active-Semi, Inc.

ACT4072

Output Voltage Setting

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

6.8

μH

6.8

μH

10

μH

15

μH

22

μH















1

V

222

.

1

V

R

R

OUT

FB2

1

FB

-





RIPPLE

OUTMAX

SW

IN

OUT

IN

OUT

K

I

f

V

V

V

V

L

-





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.

The output capacitor also needs to have low ESR to

keep low output voltage ripple. The output ripple

voltage is:

the output capacitance.

very small and does not contribute to the ripple.

Therefore, a lower capacitance value can be used

for ceramic type, typically choose a capacitance of

about 22µF.

In the case of tantalum or electrolytic type, the ripple

In that case, the output capacitor is chosen to have

sufficiently low due to ESR, typically choose a ca-

pacitor with less than 50m

Ω 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 the reverse volt-

age rating higher than the maximum input voltage.

Output Capacitor

Input Capacitor

APPLICATIONS INFORMATION

corresponding to the peak-to-peak ripple current

being 20% to 30% of the maximum output current.

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

peak inductor current is less that the 2.4A current

limit. Finally, select the inductor core size so that it

does not saturate at 2.4A.

(1)

(2)

(3)

Figure 1:

Output Voltage Setting

ESR

RIPPLE

OUTMAX

RIPPLE

R

K

L

R



OUT

2

SW

IN

LC

f

28

V



Table 1:

Typical Inductor Values

Inductor Selection