From: John Popelish
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Subject: Re: Driving an LED from 9v battery
Date: Wed, 20 Nov 2002 21:40:51 GMT
NNTP-Posting-Date: Wed, 20 Nov 2002 16:40:51 EST
Walter Harley wrote:
> I really appreciate all the help you folks have provided for me lately.
> Here's another question: I'd like to drive from one to four LED's, from a
> source with voltage as high as 12v and as low as 6v (basically, a 9v
> battery, occasionally supplemented by a poorly regulated wall-wart). I want
> constant brightness (= constant current) over the whole voltage range, and I
> want maximum possible efficiency, to preserve battery life. Need to keep
> the cost and parts count under control, need to keep the circuit physically
> small, and I need to keep audio-band noise low. The circuit will be
> incorporated into a variety of small battery-powered audio processors.
> I'm thinking that one of the many small switching converters or charge pumps
> would be right. But hunting around on the web sites I can't seem to find
> which one: most seem to be oriented to stepping up from small voltages,
> rather than stepping down, and I'm not sure I'm searching with the right
> terminology. Can someone point me in the right direction? Or, perhaps
> there's a better solution to my problem?
As long as the LED string has lower forward drop than the battery, it
is hard ot beat the efficiency of a buck regulator, with a low value
of series resistor, to provide current regulation feedback. There are
single chips that will do the whole job, except for a couple
capacitors, resistors and an inductor. If the 4 leds drop
significantly more than the minimum battery voltage, you can connect
them in parallel pairs, with a current sense resistor in series with
each string, and only use one as the feedback, with the others there
to balance the current distribution. The most expensive part will be
a low loss, high value inductor.