Re: Explanation of the whole 300 mA thing

On Dec 3, 2:15 pm, Jim Granville <no.s...@xxxxxxxxxxxxxxxxxxxxxx>
wrote:
rickman wrote:
On Dec 3, 2:09 am, Jim Granville <no.s...@xxxxxxxxxxxxxxxxxxxxxx>
The voltage budget is getting tight.
Few Display LEDs specs to 300mA, but slope of 500mv/50mA is not
uncommon, for Green.
Most matrix displays expect to be driven at lower than 16:1 duty cycle.

The dominant resistive voltage drop, should be the current limiting
resistor, not any drivers, and especially not any column drivers.

I have never seen a diode that was resistive in the forward
direction. I think you will find the forward voltage drop to be
logarithmic.

Download a kingbright data ***, and you will see at low currents
the log model is OK, at higher currents, the resistive element dominates.

As you say, these data sheets do not show the I-V curve for high
currents. In fact, the line above 30 mA seems to be extrapolated and
does not even match the end of the measured curve. Likely this is
just very rough data they collected which I don't especially trust.
The data I have seen is always logarithmic. Try this page...

http://www.planetanalog.com/features/showArticle.jhtml?articleID=202602143

Regardless, the I-V curve is only relevant if it dominate the
resistance. The dynamic resistance of the emitter follower is very
low because the dynamic resistance of the input is scaled by the gain
of the device. So there should be no problem using a 5 volt supply
with this design.

The bottom line is that we can discuss this all day, but until someone
comes up with real data, we won't know for sure just what works and
what doesn't.


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