Re: Possible High LED current with long life time

On Thu, 31 Jan 2008 16:10:35 +0000, John Devereux
<jdREMOVE@xxxxxxxxxxxxxxxxxx> wrote:

Pubs <pubuducg@xxxxxxxxx> writes:

On Jan 31, 8:54 am, Pubs <pubud...@xxxxxxxxx> wrote:
On Jan 31, 7:44 am, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:





On Wed, 30 Jan 2008 19:39:38 -0800 (PST), the renowned Pubs

<pubud...@xxxxxxxxx> wrote:
On Jan 30, 8:00 pm, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Wed, 30 Jan 2008 11:12:54 -0800 (PST), the renowned Pubs

<pubud...@xxxxxxxxx> wrote:
Hi,

  I have a application where my LED currents are switched as in a
matrix. My current LED on time is 60 us and off time is 15240 us. The
intensity is problem thus I pump a higher current in that 60 us
(applied potential is arround 9V in that period of time ) and that
gives the required intensity. LED doee not heat up since LED current
duty cyle is extreamly small (rms current is  also very small too). Is
this method will reduce the LED life time ?.
Could some one refer some articles on LED life time dependability with
current.

Thank you.

Pubs

Wow. I don't like to go much beyond 8:1 to 10:1. You're doing 250+:1.
I don't think you'll get away with it unless the peak current is not
much more than 5-10x the allowable DC current (meaning the LEDs will
be relatively dim).

Best regards,
Spehro Pefhany
My design works prefectly. IT gives almost like a continoues current
because that current pulse is repeatedly applied. My concerened was
with the life time.

Yes, yes, I know, 'prefectly'.  So what does the LED data *** say
about the conditions you are subjecting it to? For example, here is a
data *** that specifies maximum PEAK current ("Limits of Safe
Operation") at 25°C of 150mA (only 5 x the maximum DC current) for
pulses of < 10 *microseconds*).

http://www.lumex.com/pdf/SSL-LX5063ID.pdf

It should be derated above 25°C, obviously.

Pubudu

Best regards,
Spehro Pefhany
--
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- Show quoted text -

So the peak current is 150 mA at less than 10 us pulses for this LED
type. Anyway my drivers provide arround 90 mA per LED. I think I am in
the safe region.- Hide quoted text -

- Show quoted text -

My LED data *** is shown below.
http://www.jameco.com/Jameco/Products/ProdDS/183222.pdf
According to it, peak current is 500 mA for pulses less than 50 ms. I
am well withing the region.

For pulses less than *1* ms, not 50.

Actually, the data*** is strange. It gives a max DC current of 20mA,
but a max pulse of 500mA at 1/20 duty. Which is a *25* mA average, and
probably much more than that in terms of effective heating.

And is has a "continuous forward current" of "100mW"!

Very impressive part, Jameco must have access to some very high
technology suppliers. Compare a similar Avago part:
http://www.avagotech.com/assets/downloadDocument.do?id=2579

Applications
? Commercial outdoor signs
? Automotive interior lights
? Front panel indicators
? Front panel backlighting

Max DC current is 30mA (derate above 50°C) rather than 20mA (?)
Peak current is only 100mA vs 500mA for the Jameco thing.
Max Pd is 111mW vs. 100mW for the Jameco wonder.

Of course it could just be a sloppy translation and layout from a
sloppy Chinese data *** from a Guangzhou back-alley assembly outfit
that hardly knows what they are doing, but that's just conjecture.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.