From: Jonathan Kirwan
Subject: Re: Pulsing LED/High Current - Simple Circuit Question
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NNTP-Posting-Date: Wed, 08 Jan 2003 09:05:43 GMT
Organization: AT&T Broadband
Date: Wed, 08 Jan 2003 09:05:43 GMT
On Wed, 08 Jan 2003 01:09:41 -0500, Mark Klinger
>Currently, at DC, my LED has Vf of 1.7V and max current of 100mA.
Perhaps so, but just not at the same time, I think. I gather
you are working with a visible LED and it's unlikely that the Vf
will be only 1.7V when driven with 100mA.
>Since my supply is 13.8V, would it be correct that:
>13.8V-1.7V-0.2V (Sat. of Transistor)/.1A = 120ohm resistor?
I think the model must include the equivalent internal
resistance of the LED, which you don't show in the above. For
example, in some types of red LEDs, it's 21 ohms or so, with a
1.55V minimum forward voltage. In your example, this would
pencil out to:
R = (13.8V - 1.55V - 0.2V) / 0.1A - 21 ohms
= 99.5 ohms
~ 100 ohms
Similar, but you need to subtract away the internal resistance
of the LED. Also, I think your 1.7V includes a drop across that
resistor, so the actual minimum is probably somewhat less in
your case. But you'll need to find out. In the meantime, I'm
sticking with the 1.55V figure.
>Since I am able to pulse the LED at 2A with a very short pulse
>duration, would I then change to a 6 ohm resistor since:
>13.8-1.7V-0.2V/2A = 5.95 ohm?
Or more like,
R = (13.8V - 1.55V - 0.2V) / 1A - 21 ohms
= -8.95 ohms
In other words, a problem. You need to keep in mind the
internal resistance, too.
>This 2A is peak current I believe, which means that the short duration
>pulses (<1mS) should be significantly "brighter" than the DC 20mA
The PN junction's life is limited by the very slow diffusion of
the dopants. Diffusion is a strong function of die temperature,
but it's unlikely you'll wreck it this way.
Extended temperature cycling can lead to mechanical fatigue of
the bond wires or to die attachment failures, I suppose. Years
ago, it wasn't uncommon to see a 5% failure rate after 50 cycles
from -55 C to 125 C, for example.
The wires bonded to the aluminum metalization can also form
amalgams at a rate that strongly depends on temperature. At 150
C, the bond life might be as little as a few months, in fact.
At 200 C, it can be in days.
Packaging has it's own thermal expansion coefficients and the
epoxy has a "glass transition temperature" you may want to avoid
(some transition at 125 C.) Also, the epoxy is organic and can
degrade if overheated.
All of the above and more tend to suggest that you want to keep
the peak die temperature below about 115 C. And that will set
your peak current. You'll need information from the data sheet
to get an idea about what this figure is, or would be in your
setting. If they say 2A is the maximum, then look for the pulse
duration and any specified mounting, too. It should say
something about all that. If not, you need to find another way
to estimate just how much you can pulse it with.
And I don't think you'll want to shoot for 2A pulses. But then
I don't know the LED you're using, either.