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LM3402 Datasheet(PDF) 13 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part # LM3402
Description  LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board
PDF  18 Pages
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3402 Datasheet(HTML) 13 Page - National Semiconductor (TI)

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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
IN = 24V, 3 series LEDs @ 400mA
30061656
FIGURE 14. t
D + tSU Graph
13
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