CS51412 datasheet(13/20 Pages) ONSEMI

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The RMS current of the input capacitor can be calculated

using:

IRMS + IO D(1 * D)

where:

D = switching duty cycle which is equal to VO/VIN.

IO = load current.

Figure 18. Input Voltage Ripple in a Buck Converter

To calculate the RMS current, multiply the load current

with the constant given by Figure 19 at each duty cycle. It is

a common practice to select the input capacitor with an RMS

current rating more than half the maximum load current. If

multiple capacitors are paralleled, the RMS current for each

capacitor should be the total current divided by the number

of capacitors.

Figure 19. Input Capacitor RMS Current can be

Calculated by Multiplying Y Value with Maximum Load

Current at any Duty Cycle

0.2

0.4

1.0

DUTY CYCLE

0.1

0.3

0.4

0.5

0.6

0.2

0.6

0.8

Selecting the capacitor type is determined by each

design’s

constraint

and

emphasis.

The

aluminum

electrolytic capacitors are widely available at lowest cost.

Their ESR and Equivalent Series Inductor (ESL) are

relatively high. Multiple capacitors are usually paralleled to

achieve lower ESR. In addition, electrolytic capacitors

usually need to be paralleled with a ceramic capacitor for

filtering high frequency noises. The OS−CON are solid

aluminum electrolytic capacitors, and therefore has a much

lower ESR. Recently, the price of the OS−CON capacitors

has dropped significantly so that it is now feasible to use

them for some low cost designs. Electrolytic capacitors are

physically large, and not used in applications where the size,

and especially height is the major concern.

Ceramic capacitors are now available in values over 10

mF.

Since the ceramic capacitor has low ESR and ESL, a single

ceramic capacitor can be adequate for both low frequency

and high frequency noises. The disadvantage of ceramic

capacitors are their high cost. Solid tantalum capacitors can

have low ESR and small size. However, the reliability of the

tantalum capacitor is always a concern in the application

where the capacitor may experience surge current.

Output Capacitor

In a buck converter, the requirements on the output

capacitor are not as critical as those on the input capacitor.

The current to the output capacitor comes from the inductor

and thus is triangular. In most applications, this makes the

RMS ripple current not an issue in selecting output

capacitors.

The output ripple voltage is the sum of a triangular wave

caused by ripple current flowing through ESR, and a square

wave due to ESL. Capacitive reactance is assumed to be

small compared to ESR and ESL. The peak−to−peak ripple

current of the inductor is:

* P +

VO(VIN * VO)

(VIN)(L)(fS)

VRIPPLE(ESR), the output ripple due to the ESR, is equal

to the product of IP−P and ESR. The voltage developed

across the ESL is proportional to the di/dt of the output

capacitor. It is realized that the di/dt of the output capacitor

is the same as the di/dt of the inductor current. Therefore,

when the switch turns on, the di/dt is equal to (VIN − VO)/L,

and it becomes VO/L when the switch turns off. The total

ripple voltage induced by ESL can then be derived from

VRIPPLE(ESL) + ESL(

VIN

)

) ESL(

VIN * VO

)

+ ESL(

VIN

)

The total output ripple is the sum of the VRIPPLE(ESR) and

VRIPPLE(ESR).

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