From: email@example.com (Bob Wilson)
Subject: Re: Q. About ICs for Driving White LEDs
Date: Mon, 07 Oct 2002 04:56:08 -0000
Organization: Your Organization
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In article ,
>In article ,
>firstname.lastname@example.org talked about...
>> In sci.electronics.design Watson 'Atto Parsec' Name
>> > see what disadvantages they have, which don't always become apparent
>> > looking at the data sheets. I've noticed that some chips run at 1 or 2
>> > MHz, which seems high and might cause some EMI, however it probably
>> > very widespread.
>> Running at high frequency is a great advantage when you come to picking
>> the inductor- a tiny SMT part might take the place of a much larger unit
>> for a given current.
>Yeah, I've run into the stark fact that most of the chips are SMDs too. So
>that would be okay if I could get the evaluation board. But I would prefer
>that the chip was 8-pin DIP, since it will easily fit into a flashlight.
>But the problem is that it's hard to get those surface mount inductors, and
>even harder to work with them if you need to unwind a few turns.
As menstioned earlier, use the universally-available 384x-series controller.
It is in an 8-pin minidip. At these low current levels (the Luxeon products
require either 350 or 700 mA, depending on which type), a flyback is ideal.
You can wind a small toroid, or miniature RM or E-E core inductor easily.
Current sensing at this low current level can be done with a simple resistor,
and power FETs at this current level cam be similarly small.
In my last job, by the way, I designed a switchmode constant current source
to drive an RGB array of 60 Luxeon LEDs integreated into a flat panel fixture
for lighting outsides of buildings. We used RGB instead of white LEDs,
because the white ones have poor colour purity since they only contain blue
and yellow light that tricks the eye into seeing white. With RGB we not only
got a more pure white, we could also do pretty spectacular pyrotechnics with
the colours (even had a number of these luminaires mounted in a row, doing
"the wave" in colour!).