LED6000 datasheet(26/49 Pages) STMICROELECTRONICS

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Application notes - step-down conversion

LED6000

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DocID027777 Rev 2

Figure 15. LEDs small signal model

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

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

following equation:

Equation 10

The typical LED dynamic resistance, for high current LEDs, is about 0.9

 to 1 .

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

with equivalent series resistance lower than 10 m

, as a consequence the zero due to the

time constant CO * RES is in the range of 10 MHz or above.

Starting from Equation 10 it's possible to roughly estimate the required CO value:

Equation 11

In the above equation it has been assumed that the total inductor current ripple is well

approximated by the first Fourier harmonic, 8/

2 * 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 30% - 60% of the programmed

LEDs current.

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

by Equation 12.



SNS

LED



















LED

SNS

















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