> I have a few questions.  I'm looking to drive about 10 white LED's
> individually,  preferably a one chip buffer type deal.  Are there any
> such things?  It's got a PIC driving the driver, and I would like to
> be able to control the brightness of each LED w/ PWM.  I've looked
> some but I can't pin down what exactly it is that I need/am looking
> for.  Thru-hole or SMT (as long as it isn't too small) is fine. I
> think the ULN2803 may work but I'm not sure.  Any one been down this
> road before? Any gotchas about while LED's being different from
> regular LED's?

Phosphor type white LEDs (as opposed to eg RGB type) usually have a low Ipk 
to I_cont_max rating. This may be less than 2:1 for some real world 
products.

One "easy" method to do what you want, provided energy efficiency is not an 
issue, is to us a voltage drive substantially higher than LED on voltage and 
then PWM each pin via a suitable resistor. The LED's are then effectively 
driven by PWM'd current sources. A ULN2803 or similar would work fine. The 
control task is transferred to software.

If energy efficiency matters then a swiching current source per LED would 
work - this can require very few components per LED - maybe an op amp 
section or a transistor or two per LED plus an inductor per LED.


   Russell McMahon







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