DocID022745 Rev 9

19/25

STBB2

Application information

Equation 2

Equation 3

where fs is the minimum value of the switching frequency and

inductor ripple current. The peak-to-peak ripple can be set at 10% or 20% of the output

current.

The minimum inductor value for the application is the highest between Equation 2 and

Equation 3. In addition to the inductance value, the maximum current, which the inductor

can handle, must be calculated in order to avoid saturation.

Equation 4

Equation 5

where

η is the estimated efficiency. The maximum of the two above values must be

considered when the inductor is selected.

8.3

Input and output capacitor selection

It is recommended ceramic capacitors to be used with low ESR as input and output

capacitors in order to filter any disturbance present in the input line and to obtain stable

operation.

Minimum values of 10 µF for both capacitors are needed to achieve good behavior of the

device. The input capacitor must be placed as closer as possible to the device.

8.4

Layout guidelines

Due to the high switching frequency and peak current, the layout is an important design step

for all switching power supplies. If the layout is not fulfilled carefully, important parameters

such as efficiency and output voltage ripple may be compromised.

L

MAX

MAX

BUCK

MIN

I

fs

VIN

)

VOUT

VIN

(

VOUT

L

Δ

×

×

−

×

=

−

L

MIN

MIN

BOOST

MIN

I

fs

VOUT

)

VIN

VOUT

(

VIN

L

Δ

×

×

−

×

=

−

L

fs

VIN

2

)

VOUT

VIN

(

VOUT

)

/

I

(

I

MAX

MAX

OUT

BUCK

PEAK

×

×

×

−

×

+

η

=

−

L

fs

VOUT

2

)

VIN

VOUT

(

VIN

VIN

I

VOUT

I

MIN

MIN

MIN

OUT

BOOST

PEAK

×

×

×

−

×

+

×

η

×

=

−