LED2001 datasheet(23/42 Pages) STMICROELECTRONICS

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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/

π2 represents the main harmonic of the inductor current ripple (which has

a triangular shape) and

ΔI

L is the inductor current ripple.

Equation 24

so L value can be calculated as:

Equation 25

where TOFF is the OFF-time of the embedded high switch, given by 1-D.

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

the COUT value would be to meet the specification.

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

at fSW frequency.

RIPPLE s

()

------

ΔI

s ESR C

OUT

⋅⋅

()

⋅⋅

ESR

LED

⋅

() C

OUT

⋅⋅

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

ΔI

OUT

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

OFF

⋅

LED

FW_LED

100mV

⋅

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

OFF

⋅

LED

FW_LED

100mV

⋅

ΔI

------------------------------------------------------------------ TOFF

⋅

LED

FW_LED

100mV

⋅

ΔI

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

LED

FW_LED

100mV

⋅

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

–





⋅

ΔI

LED

-----------

0.5

≤

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