LM3402 datasheet(8/18 Pages) NSC

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

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Modified Application Circuit Design

Example 3

Design Example 1

•

IN = 48V (±20%)

•

Driving 3, 4, or 5 HB LEDs with V

F = 3.4V

•

F = 500 mA (typical application)

•

Estimated efficiency = 82%

•

SW = fast as possible

•

Design for typical application within t

ON and tOFF limitations

The inductor, R

ON resistor, and the RSNS resistor are calcu-

lated for a typical or average design.

• V

OUT = 3 x 3.4V + 200 mV = 10.4V

• V

OUT = 4 x 3.4V + 200 mV = 13.8V

• V

OUT = 5 x 3.4V + 200 mV = 17.2V

Calculate t

ON, tOFF & RON

Minimum ON time occurs when V

IN is at its maximum value,

and V

OUT is at its lowest value.

Calculate R

ON at VIN = 60V, VOUT = 10.4V, and set tON = 300

ns:

ON = 111 kΩ (113 kΩ) tON = 306 ns

Check to see if t

OFF minimum is satisfied.

At V

IN = 36V, VOUT = 17.2V, and RON = 113 kΩ calculate

ON:.

ON = 806 ns

OFF = 577 ns (satisfied)

30061637

FIGURE 9. Improved Average LED Current Application Circuit

LM3402 Datasheet(PDF) 8 Page - National Semiconductor (TI)

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