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ACT4060A

Wide Input 2A Step Down Converter

Unspecified 

ACT4060A

Rev 0, 28-Apr-09

APPLICATIONS INFORMATION

Output Voltage Setting

Figure 1:

Output Voltage Setting

ACT4060A

FB

VOUT

RFB1

RFB2

Figure 1 shows the connections for setting the out-

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

back resistors RFB1 and RFB2 based on the output

voltage. Typically, use RFB2 ≈ 10kΩ and determine

RFB1 from the following equation:

R FB1

=

R FB 2

⎜⎛ VOUT

⎝1.293V

− 1 ⎟⎞

⎠

(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 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:

( ) L = VOUT × VIN −VOUT

(2)

V f I K IN SW OUTMAX RIPPLE

where VIN is the input voltage, VOUT is the output

voltage, fSW is the switching frequency, IOUTMAX is the

maximum output current, and KRIPPLE is the ripple

factor. Typically, choose KRIPPLE = 30% to corre-

spond to the peak-to-peak ripple current being 30%

of the maximum output current.

With this inductor value, the peak inductor current is

IOUT × (1 + KRIPPLE/2). Make sure that this peak in-

ductor current is less that the 3A current limit. Fi-

nally, select the inductor core size so that it does

not saturate at 3A. Typical inductor values for vari-

ous output voltages are shown in Table 1.

Table 1:

Typical Inductor Values

VOUT

L

1.5V

6.8μH

1.8V

6.8μH

2.5V

10μH

3.3V

15μH

5V

22μH

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 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, how-

ever, 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:

V = I K R RIPPLE OUTMAX RIPPLE ESR

+

VIN

28 × fSW 2 LCOUT

(3)

where IOUTMAX is the maximum output current, KRIP-

PLE is the ripple factor, RESR is the ESR of the output

capacitor, fSW is the switching frequency, L is the

inductor value, and COUT is the output capacitance.

In the case of ceramic output capacitors, RESR is

very small and does not contribute to the ripple.

Therefore, a lower capacitance value can be used

for ceramic type. In the case of tantalum or electro-

lytic capacitors, the ripple is dominated by RESR mul-

tiplied by the ripple 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 electro-

lytic capacitors, choose a capacitor with less than

50mΩ 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 a reverse voltage

rating higher than the maximum input voltage.

Innovative PowerTM

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