# Driving different number of LEDs in series with a constant current driver

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?

• Does the backlight always run at the same brightness? Is it always on? Can you run all the backlights without the IC, on 12V, with the appropriate series/parallel combination and series resistors? If you need to switch the backlight, use a MOSFET on the low side. Sep 20, 2022 at 13:59
• The backlight have the brightness adjustable and can be off, it will be controlled by an ESP32 Sep 20, 2022 at 15:05

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?

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.

• The problem with running LEDs in parallel is that their Vf are not identical even from the same batch. The one with the lowest Vf will tend to hog the current and be brighter. Having said that, you have 22 backlight LEDs which you could run as 11 parallel pairs. Assuming these are distributed behind some diffusing panel you should not notice if some are brighter than others. Sep 20, 2022 at 12:55
• A small resistor in series with each LED will help share the current. If you target a 0.5 V drop at your desired LED current, there should still be enough headroom for the constant current driver. Sep 20, 2022 at 14:24
• @PeterJennings true, we (and hopefully OP) are all aware of that. But since the main idea here is to drive parallel connected LEDs, I intentionally didn't tell anything about potential problems with connecting non-identical LEDs in parallel. Even the datasheet doesn't say anything about the importance of series resistor, so I didn't bother. And I also remind that today's well-known (OSRAM, Bridgelux, CREE etc) LEDs, especially white ones, are quite good at parallel connection. Sep 21, 2022 at 6:35

Many applications indeed use parallel LEDs.

Since the voltage at the output pins must not exceed VCC, it might be a good idea for your circuit to not use LEDs in series.

• the voltage at the output pins must not exceed VCC. This is a real risk, especially if your 12V supply may come up before the 5V supply. Sep 20, 2022 at 14:25
• Good catch, I didn't see that limitation, clearly the 12V supply is a no go with this IC Sep 20, 2022 at 15:07