UM0670 datasheet(41/43 Pages) STMICROELECTRONICS

41 / 43 page

Design calculation

UM0670

41/43

Doc ID 15327 Rev 2

The number of turns for the inductor is:

Equation 21

where the inductance factor Al for the E25 core and 2 mm gap is calculated:

Equation 22

K1 = 70 (see datasheet Section 11: References and related materials: 5.)

K2 = – 0,73 (see datasheet Section 11: References and related materials: 5.)

s = E25 core air gap [mm].

The last step to complete the inductor design is to calculate the wire diameter.

Maximum inductor power dissipation is:

Equation 23

The wire resistance on the inductor is (copper wire with diameter of 0.28 mm is chosen):

Equation 24

where average turn length IN is written in the core datasheet (see datasheet Section 11:

References and related materials 5.).

The power dissipation on the wire is:

Equation 25

The power loss in the wire is much lower than the maximum power loss in the inductor, and

so a wire with a diameter of 0.28 mm is suitable for this inductor.

------

1.6

–

54.22

–

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

172

------

0.73

–

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

54.22nH

MAX LOSS

MAX

–

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

–

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

ρ×

---

π d

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

1.76

–

5172

3.14

0.028

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

17.2m

WIR E

AVR

17.2

–

17.2mW

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