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How to select O/P capacitor for LED driver in boost to ground topology?

How to select O/P capacitor for LED driver in boost to ground topology?

Infineon_Team
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The output capacitor acts as an energy tank when the diode is in reverse polarity and for this reason it is a victim of high ripple current. This component affects the bandwidth of the system, as well as, the output current ripple performance. Usually, for LED driver application multi-layer ceramic capacitors (MLCCs) with low ESR are preferred to the electrolytic capacitors.

The output voltage ripple ΔVOUT can be approximated by:

ΔVOUT = ΔIOUT * RLOAD

Assuming that the output capacitor is discharged by a constant current IOUT for a time equal to tON (related to VIN, VOUT and fSW), implies that the value of the capacitor can be calculated by:

COUT > IOUT*D/(ΔVOUT*fSW)

In addition, the equivalent series resistor (ESR) of the capacitor affects the ripple of the output voltage.

Its effect can be calculated during two phases:

  • During the capacitor discharging ΔVOUT = ESR * IOUT

  • During the charging phase ΔVOUT = ESR*(IL-IOUT)

KA-15168_Output_capacitor.png

Figure 1: Output cap

The contribution of ESR has to be negligible compared to the ripple imposed by the capacitor.

For further information check the TLD5099EP datasheet, while more information on how to design a proper solution can be found in the Z8F66213399 DRL driver with TLD5099EP B2G-AN application note.

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