User manual

2 Ps/DIV

200 mA/Div

DIM

5V/Div

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External MOSFET Dimming and Contrast Ratio

Figure 11. t

and t

(DIM Pin)

15 External MOSFET Dimming and Contrast Ratio

MOSFET Q4 and its drive circuitry are provided on the demonstration PCB (see Figure 12). When

MOSFET Q4 is turned on, it shorts LED+ to LED-, therefore redirecting the inductor current from the LED

string to the shunt MOSFET. The LM3402 / 04 is never turned off, and therefore become a perfect current

source by providing continuous current to the output through the inductor (L1). A buck converter with an

external shunt MOSFET is the ideal circuit for delivering the highest possible contrast ratio. For typical

delays and rise time for external MOSFET dimming, see Figure 13 - Figure 15.

SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current

Evaluation Board

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