30061657

FIGURE 15. t

Fast Dimming + Improved Average

Current Circuit

Using both the Improved Average LED current circuit and the

external MOSFET fast dimming circuit together has additional

benefits. If R

(f

current circuit, the switching frequency will decrease once

V

V

comes almost identical to the original unmodified circuit equa-

tion.

Setting t

t

when Q4 shunt MOSFET is on during fast dimming.

t

t

when Q2 shunt MOSFET is OFF during fast dimming.

This is an added benefit due to the fact that t

increased, and therefore the switching frequency is de-

creased, which leads to improved efficiency (see figure 16).

Inductor L1 still remains charged, and as soon as Q4 turns off

current flows through the LED string.

30061662

FIGURE 16. Improved Avg I

Linearity with Fast Dimming

Once the delays and rise/fall times have been greatly re-

duced, linear average current vs, duty cycle (D

achieved at very high dimming frequencies (f

17).

30061663

FIGURE 17. Linearity with Fast Dimming

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