> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Apptech
> Sent: 19 December 2009 10:53
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] to-92 LED driver
>
> In the long-ago time of LED
> indicator lights, LEDs were such low power and low efficiency devices
that
> you could often pulse them at 10 or even 100 x their average rating,
as
> long
> as the average current was OK. People have got used to this idea, and
so
> apparently have Zetex, but it's no longer so. with modern white LEDs.
A 20
> mA white LED will often have a 30 mA absolute maximum specification.
They
> won't die (usually) at 80 mA pulse, or 100 mA or ... but you can
expect
> their "lumen maintencnace" (light output efficiency with time) to be
from
> reduced to severely-reduced as a consequence.
> 
> ie driving a modern white LED directly from a pulsed output is a bad
idea.
> 
> Note that LED lifetime degrades with both increasing current AND
> increasing
> temperature essentially independently. ie a LED conducting more
current
> will
> have a shorter life, even if the die temperature is not increased.
(this
> radical heresy is based on LumiLeds app notes)

Hi Russell, 

What is the root cause of the degradation in the case of high current
pulses?  e.g. higher peak power dissipation from I*I*R causing die
heating or something to do with phosphor degradation due to the high
peak outputs?

Cheers

Mike

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