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L5994 - L5994A

+3.3V Inductor

To define the inductor, it is necessary to determine firstly the inductance value. Its minimum value is given by:

specific application in terms of space utilization and other practical issues like ease of mounting, availability and

so on. As to the material, the choice should be directed towards ferrite, molypermalloy or KoolM

µ, to achieve

high efficiency. These materials provide low core losses (ferrite in particular), so that the design can be concen-

trated on preventing saturation and limiting copper losses. Saturation must be avoided even at maximum peak

current:

losses can usually be neglected. A practical criterion to minimize DC resistance could be to use the largest wire

that fits the selected core.

Anyway the best solution, whenever possible, is to use an off-the-shelf inductor which meets the requirements

in terms of inductance and maximum DC current. Nowadays there is a broad range of products offered by man-

ufacturer, also for surface mount assemblies.

+5.1V Transformer

The primary winding carries the secondary power as well, thus the total primary average current is:

turns ratio in order to minimize stray parameters, the secondary is referred to the 5.1V output, and therefore the

minimum value is given by:

connected with the proper polarity (see fig. 4). The minimum primary inductance value can be expressed as:

As to the transformer realization, the considerations regarding to the +3.3V inductor can be here repeated.

L

3M IN

3.3

V

IN M AX

3.3

–

()

–

V

IN MAX

f

SW

I

OUT3

RF

⋅⋅

⋅

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

=

I

L3PK

I

OUT3

3.3

V

IN M AX

3.3

–

()

⋅

2f

SW

L

3

V

IN M AX

⋅⋅

⋅

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

+

=

I

TO T5

I

OU T 5

V

IN LIN

I

OUT12

⋅

5.1

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

+

=

η

MIN

V

IN LIN

5.1

–

V

f

+

5.1

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

=

L

5pm in

3

4

---

5.1

V

IN

5.1

–

()

2

⋅

V

IN

f

SW

I

TO T5

RF

V

IN

5.1

–

() η

–

V

IN

I

OUT12

⋅⋅

⋅

⋅

[]

⋅⋅

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

⋅

=

RF

η V

IN

I

OUT12

⋅⋅

I

TOT5

V

IN

5.1

–

()

⋅

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

>

I

5PK

I

5TO T

5.1

V

IN MA X

5.1

–

()

⋅

2f

SW

L

5P

V

IN M AX

⋅⋅

⋅

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

η I

OUT12

⋅

++

=