AN4501 datasheet(27/48 Pages) STMICROELECTRONICS

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AN4501

Selection of the output capacitor

The output capacitor determines the output voltage and current ripples.

For a current source, the choice of the output capacitor begins with the specification of the

maximum output current ripple IOUT,PP,MAX.

The capacitor voltage capability must be higher than the maximum output voltage. Note that

some margin must be taken, as this parameter has a non-negligible tolerance and varies

with the temperature and the applied DC voltage (if a MLCC capacitor is used).

6.1

Output voltage ripple

The main causes of the output voltage ripples are:

•

The charge, respectively the discharge, of the ideal capacitor without equivalent series

resistor (ESRCOUT) during tON and tOFF. The ideal capacitance is called the bulk

capacitance in the rest of the document.

•

The voltage drop caused by the output capacitor’s current ripple across ESRCOUT.

6.1.1

Contribution of the charge and discharge of the bulk capacitance

During the on-phase, the output current is exclusively delivered by the output capacitor

COUT.

During tON: iCOUT = -iOUT ~ -IOUT (seeFigure 12)

As a first approximation, we assume that the output current ripples are negligible compared

to the average value. This assumption is justified by the fact that the selection of the output

capacitor should limit the conducted emission at the output in order to fulfill stringent

specifications on the electromagnetic emissions.

The integration of the output voltage over the on-phase gives:

6.1.2

Contribution of the output capacitor ESR to the output voltage ripple

Considering that:

•

During tON, iCOUT = -iOUT ~ -IOUT and the diode is reverse biased: iD = 0

•

During tOFF, the diode is conducting and charges COUT and delivers current to the LED

strings: iCOUT ~ iD - IOUT

We can conclude that at anytime, iCOUT ~ iD - IOUT and ICOUT,PP = ID,PP = IL,PEAK (see

Figure 12).

COUT

OUT

COUT

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

CO UT

–

∼

ΔV

COUT

OUT

OU T

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

OU T

OUT

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

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