next topic »

[on:]

I'm messing around with some WS2802 LED driver chips but the datasheet is pretty poor and I can see no sensible reference to how one calculates the IREF resistor value. I have used a 10K for now and it's running cool enough but I would prefer to calculate the correct value.

[Reply #1 on:]

The datasheet shows Amps = 1.25 / Ohms * 22.  Some simple algebra gets:

Amps / 22 = 1.25 / Ohms

22 / Amps = Ohms / 1.25

(22 / Amps) * 1.25 = Ohms

For example: 20 mA -> (22 / 0.020) * 1.25 = 1375 Ohms

[Reply #2 on:]

Thanks John,

I looked again at the datasheet and found the formula. First time I had just done a search for IREF not expecting the datasheet to be calling it REXT. More haste less speed. 

[Reply #3 on:]

I looked again at the datasheet and found the formula. First time I had just done a search for IREF not expecting the datasheet to be calling it REXT. More haste less speed. 

That bugged me, too.  I had to look twice to see if there was ANOTHER pin called REXT.  Don't know why they call the pin IREF and the resistor REXT without anything to tie them together.  And on the datasheet I found the reference to the voltage coming out of the IREF pin being 1.25V was on the next page and something I had to read a few times.