ACT4060A

Rev 1, 25-Jun-09

Innovative Power

TM

- 5 -

www.active-semi.com

Copyright © 2009 Active-Semi, Inc.

(3)

OUT

2

SW

IN

LC

f

28

V

×

+

ESR

RIPPLE

OUTMAX

RIPPLE

R

K

I

V

=

1.5V

1.8V

2.5V

3.3V

5V

L

6.8μH

6.8μH

10μH

15μH

22μH

(2)

()

RIPPLE

OUTMAX

SW

IN

OUT

IN

OUT

K

I

f

V

V

V

V

L

−

×

=

(1)

⎟

⎠

⎞

⎜

⎝

⎛

−

=

1

V

293

.

1

V

R

R

OUT

2

FB

1

FB

ACT4060A

FB

APPLICATIONS INFORMATION

Output Voltage Setting

Figure 1:

Output Voltage Setting

Figure 1 shows the connections for setting the

output voltage. Select the proper ratio of the two

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:

correspond to the peak-to-peak ripple current being

30% of the maximum output current.

With this inductor value, the peak inductor current is

inductor current is less that the 3A current limit.

Finally, select the inductor core size so that it does

not saturate at 3A. Typical inductor values for

various output voltages are shown in Table 1.

Table 1:

Typical Inductor Values

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 the 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:

capacitance. In the case of ceramic output

contribute to the ripple. Therefore, a lower

capacitance value can be used for ceramic type. In

the case of tantalum or electrolytic capacitors, the

current. In that case, the output capacitor is chosen

to have sufficiently low ESR.

For ceramic output capacitor, typically choose a

capacitance of about 22µF. For tantalum or

electrolytic capacitors, 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 a reverse

voltage rating higher than the maximum input

voltage.