LM3406_09 Datasheet, PDF(17/26 Page) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

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LM3406_09 Datasheet, PDF (17/26 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

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FIGURE 9. Schematic for Design Example 2

30020325

RON and tON

A switching frequency of 450 kHz helps balance the require-

ments of inductor size and overall power efficiency, but more

importantly keeps the switching frequency below 530 kHz,

where the AM radio band begins. This design will concentrate

on meeting the switching frequency and LED current require-

ments over the nominal input range of 9V to 16V, and will then

check to ensure that the transient conditions do not cause the

LM3406 to overheat. The LM3406 will allow a small shift in

switching frequency when VIN changes, so the calculation for

RON is done at the typical expected condition where VIN =

13.8V and VO = 4.1V. The actual RON calculation uses the

high accuracy equation listed in the Appendix.

tON(VMIN) = 1090 ns

tON(VMAX) = 650 ns

OUTPUT INDUCTOR

Since an output capacitor will be used to filter some of the

LED ripple current, the inductor ripple current can be set high-

er than the LED ripple current. A value of 40%P-P is typical in

many buck converters:

ÎiL = 0.4 x 1.5 = 0.6AP-P

The minimum inductance required to ensure a ripple current

of 600 mAP-P or less is calculated at VIN-MAX:

RON = 124 kâ¦

The closest 1% tolerance resistor is 124 kâ¦. The switching

frequency and on-time of the circuit should be checked at

VIN-MIN and VIN-MAX which are 9V and 16V, respectively. The

actual fSW and tON values have been calculated with the high

accuracy equations in the Appendix.

fSW(VMIN) = 463 kHz

fSW(VMAX) = 440 kHz

LMIN = [(16 â 4.1) x 6.5 x 10-7] / (0.6) = 12.9 ÂµH

The closest standard inductor value is 15 ÂµH. The average

current rating should be greater than 1.5A to prevent over-

heating in the inductor. Inductor current ripple should be

calculated for VIN-MIN and VIN-MAX:

ÎiL(VMIN) = [(9 - 4.1) x 6.5 x 10-7] / 15 x 10-6

= 357 mAP-P

ÎiL(VMAX) = [(16 - 4.1) x 1.09 x 10-6] / 15 x 10-6

= 516 mAP-P

The peak LED/inductor current is then estimated. This calcu-

lation uses the worst-case ripple current which occurs at VIN-

MAX.

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