ADP1828YRQZ-R7 datasheet(18/32 Pages) AD

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ADP1828

Rev. 0 | Page 18 of 32

APPLICATION INFORMATION

SELECTING THE INPUT CAPACITOR

The input current to a buck converter is a pulse 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 as well as an ESR that

is low enough to mitigate input voltage ripple. For the usual

current ranges for these converters, it is good practice to use

two parallel capacitors placed close to the drains of the high-

side switch MOSFETs (one bulk capacitor of sufficiently high

current rating as calculated in Equation 2 along with a 10 μF

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:

OUT

D =

(1)

The input capacitor ripple current is approximately

)

(

RIPPLE

−

≈

(2)

where:

IL is the maximum inductor or load current.

D is the duty cycle.

OUTPUT LC FILTER

The output LC filter smoothes the switched voltage at SW, making

the dc output 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.

Choose the inductor value by the following equation:

⎥

⎦

⎤

⎢

⎣

⎡

−

OUT

(3)

where:

L is the inductor value.

fSW is the switching frequency.

VOUT is the output voltage.

VIN is the input voltage.

ΔI

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,

including the equivalent series resistance (ESR) and the equiva-

lent series inductance (ESL). The output voltage ripple can be

approximated with:

)

(

ESL

ESR

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(4)

where:

ΔV

OUT

is the output ripple voltage.

ΔI

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 4 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, as stated in the maximum ESR rating on the capaci-

tor data sheet, so this equation reduces to

ΔVOUT ≅ ΔIL ESR

(5)

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 the capacitor data sheet

usually provides some margin such that measuring the ESL is

not usually required.

In the case of output capacitors, the impedance of the ESR and

ESL at the switching frequency are small, for instance, where

the effective output capacitor is a bank of parallel MLCC capa-

citors, the capacitive impedance dominates and the ripple

equation reduces to

OUT

≅

(6)

Make sure that the ripple current rating of the output capacitors

is greater than the maximum inductor ripple current.

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