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ACT4075YH

Wide Input 2.5A Step Down Converter

Active-Semi, Inc 

ACT4075

Rev PrA, 13-Aug-07

APPLICATIONS INFORMATION

Output Voltage Setting

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 output voltage:

RFB1



RFB2





VOUT

1.222V

-1



(1)

Figure 1:

Output Voltage Setting

ACT4075

FB

VOUT

RFB1

RFB2

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)

VINfSW IOUTMAXKRIPPLE

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 = between 20% and

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

current is IOUT × (1 + KRIPPLE / 2). Make sure that this

peak inductor current is less that the 5A current

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

does not saturate at 5A.

Table 1:

Typical Inductor Values

VOUT

L

1.5V

6.8μH

1.8V

6.8μH

2.5V

6.8μH

3.3V

8.5μH

5V

15μH

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; 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.

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 RIPPLE



28



VIN

fSW 2LCOUT

(3)

where IOUTMAX is the maximum output current, KRIPPLE

is the ripple factor, RESR is the ESR resistance of the

output capacitor, fSW is the switching frequency, L is

the inductor value, COUT is the output capacitance. In

the case of ceramic output capacitors, RESR is very

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

type, the ripple is dominated by RESR multiplied by the

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.

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.

Innovative Products. Active Solutions.

-5-

www.active-semi.com

Copyright © 2007 Active-Semi, Inc.

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