STOD03AS

Application information

Doc ID 022614 Rev 1

15/24

7

Application information

7.1

External passive components

7.1.1

Inductor selection

Magnetic shielded low ESR power inductors must be chosen as the key passive

components 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 3.3 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 X5R or X7R low ESR ceramic capacitors 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 6 µF must

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

(

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fs

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VO

VO

VIN

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VO

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MIN

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MIN

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MIN

OUT

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MIN

INVERTING

PEAK

x

x

-

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+

x

x

-

=

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2

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2

(

2

2

2

)

2

(

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fs

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VO

VO

VIN

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VO

VIN

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MIN

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MIN

MIN

MIN

OUT

MIN

MIN

INVERTING

PEAK

x

x

-

x

x

+

x

x

-

=

-

η