LED's in series help needed

TC

First off, I would like to apologize if this is in the wrong location.

I am working on a car tail light that use three different LED colors, red, yellow, and white. and I want it to be controlled using a micro-processor ( BS1, BS2, or prop). I want to use a LED driver from Maxim to drive them. But I am having trouble figuring out how many LED's I can put in series that will work for my needs.
The LED's are coming from "SupperbrightLEDs.com", there 5mm High Flux led's. there specifications are:

Red: Forward voltage 2.2V, Forward current 20mA
Yellow: Forward voltage 2.2V, Forward current 20mA
White: Forward voltage 3.2V, Forward current 20mA

I was planning on using Maxim's "MAX16835" LED driver. I think it will do the job, but I don't know how many LED's I can put in series per string.

I think I have the math right, 13.8V(accounting for alternator running) / 2.2V (for red and yellow led's) = 6.27 LED's per series string. and 4.3 LED's per series string for the white LED's.

I understand that the MAX controls the current, but I don't know if it can handle 6, or 4 LED's per series string. I could not find any info on this question, so I was hoping someone here can help me.

Thanks
TC


http://www.maximintegrated.com/datasheet/index.mvp/id/5751/t/al

skylight

350ma = 17 leds total (plus half a led )

TC

I was thinking more along the lines,

4 leds in series = 20mA, and 17 series strings in parallel = 68 LEDs = 340mA

kwinn

I would suggest 4 of the red or yellow leds in series and 3 of the white ones. You will still need to add resistors to each string in order to balance the current flow through them, and to avoid having the leds go dim at idle and bright at high speed. It might be possible to go with 5 and 4 but you still need resistors to balance the currents so you may want to try it before deciding.

You may also want to measure the forward voltage of each led so you can make the strings with forward voltages as well matched as possible.

Phil Pilgrim (PhiPi)

With a boost-type LED driver, such as this one, you can drive much longer strings of series-connected LEDs.

-Phil

TC

Thanks Phil, I was looking at a boost type. I just don't like the external component count. This project is going to cost a lot (that I don't have). Just to give you an idea, I am making 10 modules that has 60 red led's, 32 Yellow led's, and 24 white led's per module. The red led's are split ( 36 led's and 24 led's), but they cannot be on the same circuit.

Thanks
TC