ADP1821

Rev. B | Page 12 of 24

APPLICATION INFORMATION

SELECTING THE INPUT CAPACITOR

The input current to a buck converter is a pulsed waveform. It

is zero when the high-side switch is off and approximately equal

to the load current when it is on. The input capacitor carries the

input ripple current, allowing the input power source to supply

only the dc current. The input capacitor needs sufficient ripple

current rating to handle the input ripple and the equivalent series

resistance (ESR) that is low enough to mitigate input voltage

ripple. For the usual current ranges for these converters, good

practice is to use two parallel capacitors placed close to the drains

of the high-side switch MOSFETs, one bulk capacitor of suffi-

ciently high current rating as calculated in Equation 1, along

with 10 μF of ceramic capacitor.

Select an input bulk capacitor based on its ripple current rating.

First, determine the duty cycle of the output with the larger load

current:

IN

OUT

V

V

D =

(1)

Second, determine the input capacitor ripple current, which is

approximately

(

)

D

D

I

I

L

RIPPLE

−

≈

1

(2)

where:

D is the duty cycle.

Use this method to determine the input capacitor ripple current

rating for duty cycles between 20% and 80%.

For duty cycles less than 20% or greater than 80%, use an input

capacitor with a ripple current rating:

(3)

OUTPUT LC FILTER

The output LC filter smoothes the switched voltage at SW,

making the output voltage an almost

dc voltage. Choose the

output LC filter to achieve the desired output ripple voltage.

Because 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.

Use the following equation to choose the inductor value:

⎥

⎦

⎤

⎢

⎣

⎡

−

Δ

×

=

IN

OUT

OUT

L

SW

V

V

V

I

f

L

1

1

(4)

where:

L is the inductor value.

ΔI

L

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

dc load current.

Choose the output bulk capacitor to set the desired output voltage

ripple. The impedance of the output capacitor at the switching

frequency multiplied by the ripple current gives the output

voltage ripple. The impedance is made up of the capacitive

impedance plus the nonideal parasitic characteristics, the ESR

and the equivalent series inductance (ESL). The output voltage

ripple can be approximated with

2

2

2

)

4

(

8

1

ESL

f

C

f

ESR

I

V

SW

OUT

SW

L

OUT

+

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

Δ

=

Δ

(5)

where:

ΔV

OUT

is the output ripple voltage.

ΔI

L

is the inductor ripple current.

ESR is the equivalent series resistance of the output capacitor

(or the parallel combination of ESR of all output capacitors).

ESL is the equivalent series inductance of the output capacitor

(or the parallel combination of ESL of all capacitors).

Note that the factors of 8 and 4 in Equation 5 would normally

be 2π for sinusoidal waveforms, but the ripple current wave-

form in this application is triangular. Parallel combinations

of different types of capacitors, for example, a large aluminum

electrolytic in parallel with MLCCs, may give different results.

Usually the impedance is dominated by ESR at the switching

frequency so this equation reduces to

ESR

I

V

L

OUT

Δ

≅

Δ

(6)

Electrolytic capacitors have significant ESL also, on the order

of 5 nH to 20 nH, depending on type, size, and geometry; and

PCB traces contribute some ESR and ESL as well. However,

using the maximum ESR rating from a capacitor data sheet

usually provides some margin such that measuring the ESL is

not usually required.