LM3402_1 Datasheet, PDF (14/18 Pages) National Semiconductor (TI) – LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board

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.
FIGURE 15. t_D + t_SD Graph
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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
(f_SW) is determined and set with the improved average LED
current circuit, the switching frequency will decrease once
V_OUT is shorted during fast dimming. With MOSFET Q4 on,
V_OUT is equal to V_FB (200 mV). The t_ON equation then be-
comes almost identical to the original unmodified circuit equa-
tion.
Setting t_ON and R_ON:
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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).
t_ON equation becomes:
when Q4 shunt MOSFET is on during fast dimming.
t_OFF equation during normal operation is:
t_OFF equation then becomes:
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FIGURE 17. Linearity with Fast Dimming
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