Application information

STOD03A

14/22

DocID17785 Rev 3

7

Application information

7.1

External passive components

7.1.1

Inductor selection

The inductor is the key passive component for switching converters.

For the step-up converter an inductance between 4.7 µH and 6.8 µH is recommended.

For the inverting stage the suggested inductance ranges from 2.2 µH to 4.7 µH.

It is very important to select the right inductor according to the maximum current the inductor

can handle to avoid saturation. The step-up and the inverting peak current can be calculated

as follows:

Equation 1

Equation 2

where

value);

η1: efficiency of step-up converter. Typical value is 0.85;

η2: efficiency of inverting converter. Typical value is 0.75.

current, at the maximum load condition, increases. A proper inductor, with a saturation

current as a minimum of 1 A, is preferred.

7.1.2

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 for the

two switching converters. A minimum real capacitance value of 2 µF must be guaranteed for

polarization, a 4.7 µF 10 V capacitor can be used to achieve the required 2 µF.

1

L

fs

V

2

)

VIN

V

(

VIN

VIN

1

I

V

I

MID

MIN

MID

MIN

MIN

OUT

MID

BOOST

PEAK

×

×

×

−

×

+

×

η

×

=

−

2

)

2

(

2

2

2

)

2

(

L

fs

VIN

VO

VO

VIN

VIN

I

VO

VIN

I

MIN

MIN

MIN

MIN

MIN

OUT

MIN

MIN

INVERTING

PEAK

x

x

-

x

x

+

x

x

-

=

-

η

2

)

2

(

2

2

2

)

2

(

L

fs

VIN

VO

VO

VIN

VIN

I

VO

VIN

I

MIN

MIN

MIN

MIN

MIN

OUT

MIN

MIN

INVERTING

PEAK

x

x

-

x

x

+

x

x

-

=

-

η