LED5000 datasheet(20/52 Pages) STMICROELECTRONICS

20 / 52 page

Application notes - buck conversion

LED5000

20/51

Doc ID 023951 Rev 1

The cut-off frequency and the phase margin are:

Equation 22

5.8

Dimming operation

The dimming input disables the switching activity, masking the PWM comparator output.

The inductor current dynamic performance when dimming input goes high depends on the

designed system response. The best dimming performance is obtained by maximizing the

bandwidth and phase margin, when possible.

As a general rule, the output capacitor minimization improves dimming performance.

Figure 12.

dimming operation example

In fact, when dimming enables the switching activity, a small capacitor value is fast charged

with low inductor value. As a consequence, the LEDs current rising edge time is improved

and the inductor current oscillation reduced. An oversized output capacitor value requires

extra current for fast charge so generating an inductor current overshoot and oscillations.

The switching activity is prevented as soon as the dimming signal goes low. Nevertheless,

the LED current drops to zero only when the voltage stored in the output capacitor goes

below a minimum voltage determined by the selected LEDs. As a consequence, a big

capacitor value makes the LED current falling time worse than a smaller one.

The LED5000 embeds dedicated circuitry to improve LED current rising edge time.

65 kHz

AM13496v1

LED5000 Scheda tecnica(PDF) 20 Page - STMicroelectronics