ADP1822

Rev. B | Page 14 of 24

APPLICATION INFORMATION

SELECTING THE INPUT CAPACITOR

The output ripple voltage is a function of the inductor ripple

current and the capacitor impedance at the switching frequency.

For high ESR capacitors, the impedance is dominated by the

ESR, while for low ESR capacitors the impedance is dominated

by the capacitance. Determine if the capacitor is high ESR or

low ESR by comparing the zero frequency formed by the

capacitance and the ESR to the switching frequency:

The input capacitor absorbs the switched input current of the

dc-to-dc converter, allowing the input source to deliver smooth

dc current. Choose an input capacitor whose impedance at the

switching frequency is lower than the input source impedance.

Use low equivalent series resistance (ESR) capacitors, such as

low ESR tantalum, ceramic, or organic electrolyte (such as

Sanyo OS-CON) types. For all types of capacitors, make sure

that the current rating of the capacitor is greater than the input

rms ripple current, which is approximately

1

−

×

×

≅

OUT

IN

IN

OUT

LOAD

IN_RMS

V

V

V

V

I

I

(1)

OUTPUT LC FILTER

The output LC filter smoothes the switched voltage at SW,

making the output an almost dc voltage. Choose the output LC

filter to achieve the desired output ripple voltage. Since the

output LC filter is part of the regulator negative-feedback

control loop, the choice of the output LC filter components

affects the regulation control-loop stability.

Choose an inductor value such that the inductor ripple current

is approximately 1/3 of the maximum dc output load current.

Using a larger value inductor results in a physical size larger

than required, and using a smaller value results in increased

losses in the inductor and/or MOSFET switches.

Choose the inductor value by

⎥

⎦

⎤

⎢

⎣

⎡

−

=

IN

OUT

OUT

L

SW

V

V

V

I

f

L

1

)

Δ

)(

(

1

(2)

where:

L

is the inductor value.

is the switching frequency.

is the output voltage.

is the input voltage.

ΔI

L

is the inductor ripple current, typically 1/3 of the maximum

dc load current.

Choose the output capacitor to set the desired output voltage

ripple. The ADP1822 functions with output capacitors that have

both high and low ESR. For high ESR capacitors, such as tantalum

or electrolytic types, many parallel connected capacitors may be

required to achieve the desired output ripple voltage. When

choosing an output capacitor, consider ripple current rating,

capacitance, and ESR. Make sure that the ripple current rating is

()(

)

ESR

C

f

OUT

ESRZ

π

2

1

=

(3)

where:

is the frequency of the output capacitor ESR zero.

is the output capacitance.

ESR

is the equivalent series resistance of the capacitor.

is high ESR, and the ESR dominates the impedance at the switching

capacitor is low ESR, and the impedance is dominated by the

capacitance at the switching frequency.

When using capacitors whose impedance is dominated by ESR

at the switching frequency (such as tantalum or aluminum

electrolytic capacitors), approximate the output voltage ripple

current by

)

(

Δ

Δ

ESR

I

V

L

(4)

where:

ΔV

OUT

is the output ripple voltage.

ΔI

L

is the inductor ripple current.

ESR is the total equivalent series resistance of the output

capacitor (or the parallel combination of ESR of all parallel-

connected output capacitors).

Make sure that the ripple current rating of the output capacitor(s) is

greater than the maximum inductor ripple current.

For output capacitors whose ESR is much lower than the

capacitive impedance at the switching frequency, the capacitive

impedance dominates the output ripple current. In this case,

determine the ripple voltage by

()( )

SW

OUT

L

OUT

f

C

I

V

8

Δ

Δ

≅

(5)

where:

C

is the output capacitance.

OUT