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This mod/circuit is supplied in its entirety
by Ned K. Thanks Ned!
Luxeon Star Flashlight
This is a design using a Linear Technology LT1308B (datasheet
at http://www.linear-tech.com/prod/datasheet.html?datasheet=415)
switchmode converter to power a Luxeon Star high-output LED.
It drives a constant current through the LED, to avoid damaging
it and to track the best power point over temperature and manufacturing
variations.
I've assembled one and tried it out, getting 80% efficiency
with two AA alkaline cells and 310mA output. See Flashlight
Assembly Photos for more details.
It should work (with one LED) down to about 1V input, or up
to 10V input. Step-up only. Maximum output voltage is 36V, current
is set by the resistors R4 and R5 that are in series with the
LEDs (200mV across this resistor at desired operating current).
Though I only used a 16V output capacitor, so this is really
its output voltage limit.
The secret here is to use the low-battery comparator (LBI and
LBO pins) to sense the current. When the voltage across R5 is
lower than 200mV, the LBO pin is pulled low, causing the output
voltage to increase. So the output is a sawtooth of a few mA
around the current setpoint.
According to the SPICE simulation, it's about 90% efficient
when going from a 6V input to a 10V output (3 Luxeon Star LEDs
in series).
This board is 0.8 by 1.0 inches, and is set up for an external
LED:
This is set up for a Coleman double AA flashlight mod; it's
0.820 inches in diameter:
Many thanks to Ned K. for sharing this project
with FlashlightReviews.com and its visitors!
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