Explanation of the whole 300 mA thing

On Dec 3, 2:15 pm, Jim Granville <no.s...@designtools.maps.co.nz>
wrote:
> rickman wrote:
> > On Dec 3, 2:09 am, Jim Granville <no.s...@designtools.maps.co.nz>
> >>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 sheet, 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.

rickman wrote:
> On Dec 3, 2:15 pm, Jim Granville <no.s...@designtools.maps.co.nz>
> wrote:
> 
>>rickman wrote:
>>
>>>On Dec 3, 2:09 am, Jim Granville <no.s...@designtools.maps.co.nz>
>>>
>>>>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 sheet, 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

:) Well, you can choose what you like - I'll take the LED device 
suppliers data sheet (and experience) over a IC guy's model, anyday.

Have a look at the KAD1-9090ZG9ZC - there you can see a straight line
on a LOG graph at low currents, and then at higher currents the 
slope-resistance dominates, and it morphs to a curve on log paper
- with this LED, they do spec Min and Max, because they expect you to 
drive it hard, and to need a proper power/voltage budget.

> 
> 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 Std Green LED example I quoted has a RSlope of 10 ohms, and so is 
1V/100mA, that means it WILL be a real struggle to get the target 300mA
even with NO additional resistance.

Probably OK for a student project, but not for volume production.
That's why I suggested a separate LED power rail and control.

The OP's design is relatively small, I've seen the current budgets get 
to 100+ Amps on LED displays :)

-jg