AN4501 datasheet(29/48 Pages) STMICROELECTRONICS

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AN4501

Selection of the output capacitor

The resulting output current ripple is:

Equation 21:

6.3

Calculation of the output capacitor

Ceramic capacitors are recommended. As they have a low ESR, we can first select the

output capacitance, neglecting the contribution of ESRCOUT. The Equation 21 becomes:

Extracting COUT yields:

Equation 22:

Once the output capacitor is chosen based on Equation 22, its ESR is known, and the

contribution of the ESR to the output current ripple can be calculated, so that the

assumption can be confirmed.

OUT PP

ΔV

COUT

ΔV

COUT,ESR

DOUT

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

LED

DLED

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

OUT

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

ESR

COUT

L PEAK







OUT PP MAX

LED

DLED

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

OU T

MAX

OUT MIN

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

∼

OU T MIN

LED

DLED

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

OUT

MAX

OU T PP MAX

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

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