DocID18279 Rev 5

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ST1CC40

Application information

37

The LED ripple current can be calculated as the inductor ripple current ratio flowing into the

output impedance using the Laplace transform (see Figure 11):

Equation 19

where the term 8/

a triangular shape) and

Equation 20

so L value can be calculated as:

Equation 21

As a consequence, the lower the inductor value (so the higher the current ripple), the higher

A general rule to dimension L value is:

Equation 22

Finally the required output capacitor value can be calculated equalizing the LED current

ripple specification with the module of the Fourier transformer (see Equation 19) calculated

Equation 23

Example (see Section : Example):

The output capacitor value must be dimensioned according to Equation 23.

Finally, given the selected inductor value, a 2.2 µF ceramic capacitor value keeps the LED

current ripple ratio lower than 2% of the nominal current. An output ceramic capacitor type

(negligible ESR) is suggested to minimize the ripple contribution given a fixed capacitor

value.





8



2

------

I

L



+





1s



++

 C

OUT



+

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

=

I

L

L



+



L



==

L

+



I

L



+



I

L

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

1

+



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

–







==

I

L

-----------

0,5











=