ACT4072SH-T データシートの表示（PDF） - Active-Semi, Inc

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ACT4072SH-T

Wide Input 2A Step Down Converter

Active-Semi, Inc 

Active-Semi

ACT4072

Rev2, 27-May-08

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:

R FB1

=

R FB2

⎜⎛ VOUT

⎝ 1.222 V

- 1 ⎟⎞

⎠

(1)

Figure 1:

Output Voltage Setting

ACT4072

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)

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 = 20% to 30%

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

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

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.

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 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, 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 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.

Output Capacitor

The output capacitor also needs to have low ESR to

keep low output voltage ripple. The output ripple

voltage is:

R = L K R RIPPLE

OUTMAX RIPPLE ESR

+

VIN

28fSW 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.

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

is dominated by RESR multiplied by the ripple current.

In that case, the output capacitor is chosen to have

sufficiently low due to ESR, typically choose a ca-

pacitor 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 PowerTM

-5-

www.active-semi.com

Copyright © 2008 Active-Semi, Inc.

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