AN4501 datasheet(20/48 Pages) STMICROELECTRONICS

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Selection of the inductor

AN4501

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DocID026394 Rev 1

Equation 7:

This shows that the maximum value of the inductor peak-to-peak current is reached for a

duty cycle of D50 = 50%.

4.4

Inductor current ripple ratio

The inductor current ripple ratio r is defined as the ratio between the peak-to-peak current

ripple and the average current.

Equation 8:

Increasing the value of the inductance, we reduce the inductor current ripple and the output

voltage ripple, as we will see in the section Section 6.1.2. In general, the max allowed

inductor current ripple ratio is optimal for a value in the range of 0.3 to 0.5, from the

standpoint of the cost / current ripple.

Indeed, reducing r to a value much lower than 0.3 leads a very large inductor size.

Increasing r to a value which is much higher than 0.5, does not lead to a significant size

reduction (see Appendix C: Document management). Therefore setting r to 0.4 or 0.5 is a

good starting point.

Once the maximal inductor peak-to-peak current is fixed, we can estimate the minimum

required inductance value, using Equation 6 applied at VIN,MIN which corresponds to a duty

cycle DMAX.

Equation 9:

4.5

Inductor peak current

The inductor peak current must be calculated to make sure that in all cases, the inductor

saturation current is not reached.

The inductor peak current, ILPEAK is given by:

LPP

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

OU T

12D

–

()

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

LPP

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

L,PP,MAX

LM AX

OUT

MAX

–

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

MIN

OU T

MAX

–

()

L,PP,MAX

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

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