LED2000DR datasheet(19/40 Pages) STMICROELECTRONICS

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DocID023432 Rev 4

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LED2000

Application notes

6.6

Dimming operation

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

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

system response. The best dimming performance is obtained maximizing the bandwidth

and phase margin, when it is possible.

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

Figure 11. 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 certain inductor current 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 LED2000 embeds dedicated circuitry to improve LED current falling time.

As soon as the dimming input goes low, the low-side is kept enabled to discharge COUT until

the LED current drops to 60% of the nominal current. A negative current limitation (-1 A

typical) protects the device during this operation (see Figure 12).

AM12902v1

LED2000DR Datasheet(PDF) 19 Page - STMicroelectronics