STBB3JCC datasheet(19/27 Pages) STMICROELECTRONICS

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DocID025425 Rev 5

19/27

STBB3JCC

Application information

8.3

Inductor selection

The inductor is the key passive component for switching converters. With a buck-boost

device, the inductor selection must take into consideration the following two conditions in

which the converter works:

•

as buck region at the maximum input voltage

•

as boost region at the minimum input voltage

Two critical inductance values are then obtained according to the following formulas:

Equation 3

Equation 4

where fs is the minimum value of the switching frequency and ΔIL is the inductor ripple

current. The amplitude of the inductor ripple current is typically set between 20% and 40%

of the maximum inductor current. To guarantee an inductor ripple current always lower than

the selected value

ΔIL, the higher value between LMIN_BUCK and LMIN_BOOST have to be

chosen. In addition to the inductance value, also the maximum current which the inductor

can handle must be calculated in order to avoid saturation.

Equation 5

Equation 6

where

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

considered when selecting the inductor.

8.4

Input and output capacitor selection

It is recommended to use ceramic capacitors 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, CIN and COUT, are needed to achieve good

behavior of the device. The input capacitor must be placed as close as possible to the

device.

An R-C filter is added to VINA pin (R3-C5 Figure 30 and Figure 31) to assure a clean input

voltage to the internal logic block.

MIN_BUCK

OUT

IN_MAX

OUT

–

()

IN_MAX

ΔI

---------------------------------------------------------------------

MIN_BOOST

IN_MIN

OUT

IN_MIN

–

()

OUT

ΔI

-------------------------------------------------------------------------

PEAK_BUCK

OUT

------------

OUT

IN_MAX

OUT

–

()

IN_MAX

---------------------------------------------------------------------

PEAK_BOOST

OUT

η V

IN_MIN

---------------------------------



 VIN_MIN

OUT

IN_MIN

–

()

OUT

-------------------------------------------------------------------------

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