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Copyright © 2007 Active-Semi, Inc.

ACT4075

Rev PrA, 13-Aug-07

FB

ACT4075

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

6.8μH

6.8μH

6.8μH

8.5μH

15μH















1

V

222

.

1

V

R

R

OUT

FB2

1

FB

-

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 large current flows in and out of this

capacitor during switching, its ESR also affects effi-

ciency.

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 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:

small and does not contribute to the ripple. There-

fore, a lower capacitance value can be used for ce-

ramic type. In the case of tantalum or electrolytic

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.

OUT

2

SW

IN

RIPPLE

RIPPLE

OUTMAX

RIPPLE

LC

f

28

V

R

K

I

V







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

Inductor Selection

Table 1:

Typical Inductor Values

Input Capacitor

APPLICATIONS INFORMATION

30% to correspond to the peak-to-peak ripple current

being a percentage of the maximum output current.

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

peak inductor current is less that the 5A current

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

does not saturate at 5A.

(1)





RIPPLE

OUTMAX

SW

IN

OUT

IN

OUT

K

I

f

V

V

V

V

L

-





(2)

(3)

Figure 1:

Output Voltage Setting