LED6000 datasheet(16/49 Pages) STMICROELECTRONICS

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Functional description

LED6000

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DocID027777 Rev 2

4.3

Digital dimming

The switching activity is inhibited as long as the DIM pin is kept below the VDIML threshold.

When the DIM is asserted low, the HS MOS is turned-off as soon as the minimum on-time is

expired and the COMP pin is parked close to the maximum ramp peak value, in order to

limit the input inrush current when the IC restarts the switching activity. The internal

oscillator and, consequently, the IC quiescent current are reduced only if the DIM is kept low

for more than TDIMTO timeout.

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 in

terms of the shorter LEDs current rising time and reduced inductor peak current.

An oversized output capacitor value requires an extra current for the fast charge so

generating an inductor current overshoot and oscillations.

Refer also to Section 5.2 on page 25 for output capacitor design hints.

The dimming performance depends on the current pulse shape specification of the final

application.

Figure 8. Dimming operation

The ideal current pulse has rectangular shape; however, in any case it degenerates into

a trapezoid or, at worst, into a triangle, depending on the ratio (tRISE + tFALL)/ TLED.

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