Jul. 12. 2012 / Preliminary

9/13

* This specifications are subject to be changed without notice

http://www.ad-tech.co.kr

ADT6780

Thermally enhanced Low V

Input Capacitor

The input capacitor is used to filter out discontinuous, pulsed input current and to maintain input voltage

stable. Therefore input capacitor should be able to supply the AC current to the step-down converter. Its

input ripple voltage can be estimated by:

where, CIN is input capacitor value.

The voltage rating of input capacitor must be greater than the maximum input voltage plus ripple voltage.

Since the input capacitor absorbs the input switching current, it requires an proper ripple current rating. The

RMS current in the input capacitor can be approximated by:

The worst-case condition occurs at V

approximately half of the I

current rating higher than I

capacitors because of their low ESR and high current rating. When selecting ceramic capacitors, X5R or

X7R type dielectric ceramic capacitors should be used for their better temperature and voltage

characteristics. For most applications, a 10㎌ ceramic capacitor is sufficient.

Output Capacitor

The output capacitor is required to maintain the DC output voltage. In a step-down converter circuit, output

ripple voltage is determined by the inductor value, switching frequency, output capacitor value and ESR.

That is:

where,

C

ESR is the equivalent series resistance of the output capacitor.

Low ESR capacitors are preferred to keep the output voltage ripple low. When low ESR ceramic capacitor is

used as output capacitor, its ESR value can be waived. So, the impedance at the switching frequency is

dominated by the capacitance. Therefore the output voltage ripple is:

On the other hand, in the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the

switching frequency. In this case, the output voltage ripple is:

In a step-down converter, output capacitor current is continuous. Usually, the ripple current rating of the

output capacitor is not concerned because of its low ripple current. For most applications, a 10㎌ ceramic

capacitor is sufficient.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

IN

OUT

IN

OUT

IN

SW

OUT

IN

V

V

1

V

V

C

F

I

ΔV

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

IN

OUT

IN

OUT

OUT

CIN_RMS

V

V

1

V

V

I

I

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

=

O

SW

L

OUT

C

F

8

1

ESR

ΔI

ΔV

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

×

=

O

SW

L

OUT

C

F

8

1

ΔI

ΔV

(

)

ESR

ΔI

ΔV

L

OUT

×

=