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Design Practice with Blue or White LEDs


BradLevy

This set of questions stems from footnotes I've seen on data sheets for blue or white LEDs.

One is a footnote seen sometimes on the Reverse Voltage specification:

Note: Reverse Voltage can't be continued operating

Another I've seen on the graph of luminous intensity vs forward voltage, recommending against operation below a certain forward voltage.

These footnotes are easily overlooked if one isn't reading the data sheet closely.

So my questions for an expert in LEDs:

What are the reasons behind these warnings?

What I do know:
I realize that spectral response can vary with applied forward voltage, so might not meet the data sheet specs at low Vf. But are there other consequences behind the warning against operation below a certain forward voltage?

As far as the reverse voltage warning, I remember when blue LEDs first came out hearing that they were more easily damaged electrically than previous LED types. But I'm not sure whether to interpret the warning I quoted from the data sheet as an admonition to be very careful to avoid anything over the 5V Vr at which the leakage is 10 uA, or whether there is some slow incremental damage issue even at voltages below the 5V Vr spec in the data sheet. There are common circuits where two LEDs are wired back-to-back, so the polarity of the applied voltage determines which of the two LEDs gets lit. Are these a no-no with blue or white LEDs if you are concerned about long term reliability? Or does the presence of the opposite-polarity LED insuring the reverse voltage applied to the other LED doesn't exceed 3 volts keep it low enough to avoid damage from the reverse voltage? I had assumed the latter until I noticed the data sheet footnote.

I note there are a few 2-lead bipolar LEDs with blue as one of the two colors available. I don't know if those have any special formulation or protection.

I did some web searching on these issues, but couldn't find much authoritative in plain practical language. I'd love to hear some input from an expert, so I can better understand and protect against the underlying issues.

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