(3)

IN

RIPPLE

V

D (1 D)

I

fs L

´

´

-

=

´

(4)

The maximum inductor ripple current occurs when the duty cycle (D) is 0.5 or approximately 0.5. Usually

inductor ripple is designed in the range between 20% and 40% maximum charging current as a trade-off

between inductor size and efficiency for a practical design.

9.2.1.2.4 Input Capacitor

Design input capacitance to provide enough ripple current rating to absorb the input switching ripple current.

Worst case RMS ripple current is half of the charging current when the duty cycle is 0.5. If the converter does not

to 50% and can be estimated using Equation 5.

CIN

CHG

I

I

D (1 D)

=

´

´

-

(5)

A low ESR ceramic capacitor such as X7R or X5R is preferred for the input decoupling capacitor and should be

placed as close as possible to the drain of the high-side MOSFET and source of the low-side MOSFET. The

voltage rating of the capacitor must be higher than the normal input voltage level. A rating of 16-V or higher

capacitor is preferred for 5-V input voltage.

9.2.1.2.5 Output Capacitor

Ensure that the output capacitance has enough ripple current rating to absorb the output switching ripple current.

RIPPLE

COUT

RIPPLE

I

I

0.29

I

2

3

=

»

´

´

(6)

The output capacitor voltage ripple can be calculated as follows:

OUT

OUT

O

2

IN

V

V

V

1

V

8LCfs

æ

ö

D

=

-

ç

÷

è

ø

(7)

At certain input and output voltage and switching frequency, the voltage ripple can be reduced by increasing the

output filter LC.

BQ25302

SLUSCZ3 – OCTOBER 2020

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