The current in the output capacitor has a triangular waveform, which generates a voltage ripple across it. This

ripple is due to the capacitive component (charge and discharge of the output capacitor) and the resistive

component (due to the voltage drop across its ESR). So the output capacitor must be selected in order to have a

voltage ripple compliant with the application requirements.

The allowed LED current ripple (∆ILED,PP) is typically 2% to 5% of the LED DC current.

Figure 15. LED small signal model

V LED+

L

LX

R

V LED+

L

R

LX

D

RD

SNS

R

SNS

C

O

C

O

RES

RES

RDC

RDC

Based on the small signal model typically adopted for LEDs (as shown in Figure 15. LED small signal model), the

amount of the inductor current ripple which flows through the LEDs can be estimated by the following equation:

ΔILEDs =ΔILs ⋅ 1+sC0RES

1+sC0⋅(RES+RSNS+N⋅Rd)

(10)

The typical LED dynamic resistance, for high-current LEDs, is about 0.9 Ω to 1 Ω.

The output capacitor, CO, is typically an MLCC ceramic capacitor in the range of 1 µF and with equivalent series

resistance lower than 10 mΩ, as a consequence the zero due to the time constant CO*RES is in the range of 10

MHz or above.

Starting from Eq. (10) it is possible to roughly estimate the required CO value:

C0≥ 8π2⋅ΔIL,PP⋅

1

2π⋅fSW⋅ RSNS+N⋅Rd ⋅ΔILED,PP

(11)

In the above equation it has been assumed that the total inductor current ripple is well-approximated by the first

Fourier harmonic, 8/π2*∆IL,PP, due to the inductor current triangular shape.

The inductance value fixes the current ripple flowing through the output capacitor and LEDs. So the minimum

inductance value, in order to have the expected current ripple, must be selected.

The rule to fix the current ripple value is to have a ripple at 40%-60% of the programmed LEDs current.

In the continuous conduction mode (CCM), the required inductance value can be calculated by the following

equation:

L=VOUT⋅ 1−VOUTVIN

ΔIL⋅FSW

(12)

In order to guarantee a maximum current ripple in every condition, Eq. (12) must be evaluated in case of

maximum input voltage, assuming VOUT fixed.

Increasing the value of the inductance helps to reduce the current ripple but, at the same time, strongly impacts

the converter response time in case of high-frequency dimming requirements. On the other hand, with lower

inductance value the regulator response time is improved but the power conversion efficiency is impacted and the

output capacitor must be increased to limit the current ripple flowing through the LEDs.

As usual, the L-CO choice is a trade-off among multiple design parameters.

ALED6000

Output capacitor and inductor selection

DS13018 - Rev 4

page 20/45