External MOSFET Dimming and

Contrast Ratio

Refer to figure 12. MOSFET Q4 and its drive circuitry are pro-

vided on the demonstration PCB. 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. Refer to figures 13-15 for typ-

ical delays and rise time for external MOSFET dimming.

30061654

FIGURE 12.

30061655

FIGURE 13. V

30061656

FIGURE 14. t

13

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