After two days of searching, I'm a bit lost when trying to design an LED with a constant current driver IC.
In my circuit, I have LED indicators I want to control independently and an LED backlight which will light up at the same time.
I understood that putting LEDs in series will use the same current for all the string but the voltage will drop for each LED with it's Vf. If I put 4 yellow common LEDs in series, I've got 20mA consumption and if the power source is 12V and Vf is 2.1V, the final voltage will be 3.6V.
With all of that, I searched for a constant current driver chip and found the IS31FL3236A with 36 channels and 38mA max per channel. I have the idea to use it like this:
When reading the datasheet in details, I found page 11 on "Current setting" section the following sentence:
When channels drive different quantities of LEDs, adjust the maximum output current according to quantity of LEDs to ensure average current of each LED is the same.
For example, set REXT = 3.3kΩ then IMAX = 23mA. If OUT1 drives two LEDs and OUT2 drives four LEDs, set the SL bit of LED Control Register (26h) to “01” and SL bit of LED Control Register (27h) to “00”. So the current of OUT1 is IOUT1 = I MAX /2 = 11.5mA and the current of OUT2 is IOUT2 = I MAX = 23mA. The average current of each LED is the same.
Here I'm lost. Why does the datasheet talk about adapting the current with the number of LEDs if the number of LEDs doesn't change the current in the string? Are they talking about putting the LEDs in parallel?
Do you think the circuit I have in mind will work or do I need two drivers, one for the indicator LEDs and one for the backlight because of the mix of 5V and 12V?
Best Answer
The idea behind average current adjustment per channel is to keep the brightness the same across the channels. Remember that the datasheet doesn't say anything about series-connected LEDs because it assumes that any channel will drive a single LED only (check the typical application schematic at p.1) therefore multiple LEDs driven by a channel should be connected in parallel .
So, if one channel drives twice the number of "parallel-connected LEDs" that another channel drives then its drive current should be twice as well so that, assuming the LEDs will share the current equally, each LED has equal current hence equal brightness.
If brightness difference is not a concern then you don't need to make any adjustment for a channel. Just beware the maximum current and voltage ratings.