AN4501 datasheet(28/48 Pages) STMICROELECTRONICS

28 / 48 page

Selection of the output capacitor

AN4501

28/48

DocID026394 Rev 1

Figure 12. Typical current waveforms of the output capacitor current

Equation 18:

The contribution ESRCOUT to the output voltage ripple is:

Equation 19:

6.1.3

Total output voltage ripple

Summing both contributions, we obtain:

The substitution of IL,PP by its expression from Equation 11 yields:

Equation 20:

6.2

Output current ripple

The output current ripple is the ratio between the total output voltage ripple and the dynamic

resistance of the LED string (note that RSENSE and RONM2 can be neglected compared to

the dynamic resistance of the LED string).

RD,OUT ~ NbLED x RD,LED where NbLED is the number of LEDs in the string and RD,LED is

the dynamic resistance one LED at the considered output current.

CO UT PP

DPP

L PEAK

OU T

–

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

OUT

–

()

×η

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

ΔV

COUT,ESR

ESR

CO U T

L PEAK

OUT PP

ΔV

COUT

ΔV

CO UT ESR

OUT

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

ESR

COUT

L PEAK

OUT PP

OU T

OUT

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

ESR

COUT

OUT

–

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

OU T

–

()

×η

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

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