I am building a light which is using PWM waveforms to control the brightness etc. I have 2 LED arrays, 25 each, so in total 50. These are configured to 5x5 (5 in series, 5 in parallel).

I am using an LED driver chip. This has a resistor to set the amount of current to flow through the LED's.

I am just not sure how much current to run through the LED's.

1 type has a max DC forward current of 50mA. The other is 25mA max DC forward current.

These are the absolute maximum ratings.

But the Electrical Optical Characteristics give

30mA at 3.4V, the other with 20mA at 3.2V

Shall I use these values, or the maximum ones?

 

Use the 20mA rating; it's the lowest common denominator.

If you try to use the higher current, you will shorten the life of the display drastically.

 

At the electrical-optical characteristics I think they say the light intensity at various current levels. You can drive the leds with their maximum continuous current safely. Take care not to confuse it with the maximum pulsed current.

 

I think I could drive them at the max current perfectly fine, but I shall go with the 30mA and 20mA ratings. If I get a grilling by my lecturers I can just say I followed the data sheet. Thanks for the input!

 

Well, if you say that you followed the datasheet, I would give you a failing grade - because that's what would happen to the display; rapid failure.

Err on the side of caution. The display might not be as bright, but it will last for a long time.

 

Actually with today's modern LEDs most 20 ma max rated devices seem to be almost as bright at 10ma to me and usable as a simple indicator lamp even down to 1-2ma.

No reason to push the design to rated maximum as you really won't gain much brightness and the lifetime rating will increase dramatically if used at less then max ratings. If you really need brighter then either go with brighter LEDs or learn about pulsed operation at higher current rates.

Good luck

Lefty