ADP1829ACPZ-R7 datasheet(16/32 Pages) AD

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ADP1829

Rev. 0 | Page 16 of 32

APPLICATIONS 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 and also 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 sufficiently 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.

If both Channel 1 and Channel 2 maximum output load

currents are about the same, the input ripple current is less than

half of the higher of the output load currents. In this case, use

an input capacitor with a ripple current rating greater than half

of the highest load current.

RIPPLE

(1)

If the Output 1 and Output 2 load currents are significantly

different (if the smaller is less than 50% of the larger), then the

procedure in Equation 1 yields a larger input capacitor than

required. In this case, the input capacitor can be chosen as in

the case of a single phase converter with only the higher load

current, so first determine the duty cycle of the output with the

larger load current.

OUT

D =

(2)

In this case, the input capacitor ripple current is approximately

)

(

RIPPLE

−

≈

(3)

where IL is the maximum inductor or load current for the

channel and 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 ripple current rating IRIPPLE > 0.4 IL.

Selecting the Output LC Filter

The output LC filter attenuates the switching voltage, making

the output an almost dc voltage. The output LC filter charac-

teristics determine the residual output ripple voltage.

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 is required, and using a smaller value results in increased

losses in the inductor and MOSFETs.

Choose the inductor value using the equation

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

OUT

(4)

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, the

equivalent series resistance (ESR), and the equivalent series

inductance (ESL). The output voltage ripple can be approxi-

mated with

⎟

⎠

⎞

⎜

⎝

⎛

ESL

ESR

OUT

(5)

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 5 would normally

be 2π for sinusoidal waveforms, but the ripple current waveform

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

capacitor data sheet, so this equation reduces to

ESR

OUT

≈

(6)

Electrolytic capacitors have significant ESL also, on the order of

5 nH to 20 nH, depending on type, size, and geometry. 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.

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