DocID024346 Rev 1

23/42

LED2001

Application information

Figure 15. Equivalent circuit

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 23

where the term 8/

a triangular shape) and

Equation 24

so L value can be calculated as:

Equation 25

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 26

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

ripple specification with the module of the Fourier transformer (see Equation 23) calculated

I

()

Δ

8

π

2

------

ΔI

L

1

s ESR C

OUT

⋅⋅

+

()

⋅⋅

1s

R

S

ESR

n

LED

R

LED

⋅

++

() C

OUT

⋅⋅

+

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

=

ΔI

L

V

OUT

L

-------------- T

OFF

⋅

n

LED

V

FW_LED

100mV

+

⋅

L

------------------------------------------------------------------ T

OFF

⋅

==

L

n

LED

V

FW_LED

100mV

+

⋅

ΔI

L

⋅

n

LED

V

FW_LED

100mV

+

⋅

ΔI

L

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

1

n

LED

V

FW_LED

100mV

+

⋅

V

IN

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

–





⋅

==

ΔI

L

I

LED

-----------

0.5

≤