LM3402 datasheet(14/18 Pages) NSC

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30061657

FIGURE 15. t

D + tSD Graph

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

ON and the converter's switching frequency

SW) is determined and set with the improved average LED

current circuit, the switching frequency will decrease once

OUT is shorted during fast dimming. With MOSFET Q4 on,

OUT is equal to VFB (200 mV). The tON equation then be-

comes almost identical to the original unmodified circuit equa-

tion.

Setting t

ON and RON:

ON equation becomes:

when Q4 shunt MOSFET is on during fast dimming.

OFF equation during normal operation is:

OFF equation then becomes:

when Q2 shunt MOSFET is OFF during fast dimming.

This is an added benefit due to the fact that t

OFF is greatly

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

LED Circuit + Fast Dimming

Linearity with Fast Dimming

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

duced, linear average current vs, duty cycle (D

DIM) can be

achieved at very high dimming frequencies (f

DIM) (see figure

17).

30061663

FIGURE 17. Linearity with Fast Dimming

LM3402 bảng dữ liệu(PDF) 14 Page - National Semiconductor (TI)