# Constant-current driving with LED Driver and Boost DC/DC converter

Novice here, sorry if I'm missing something obvious.

I'm trying to drive 4 strips of 6 low-power RGB LEDs each (APF3236LSEEZGKQBKC) on a 3.3V battery. I have a 12-channel LED Driver (TLC6C5912) which seem perfect for the job.

However, voltage is an issue, as each green or blue channel could need up to about 20V depending on the brightness I settle for. A DC/DC boost converter seems to be the way to go to get 20V bursts out of my little battery, but from what I know, I should drive LEDs by constant current, not constant voltage, in order to account for brightness variations and possible damage due to temperature rises. I was thinking something like the TPS61096A.

Should I use something else than a DC/DC boost converter? Or add something to it to control the current?

from what I know, I should drive LEDs by constant current, not constant voltage

Isn't that why you chose the TLC6C5912 LED driver: -

So, choose a voltage boost converter to give you circa 30 volts and you should be good to go. The one you indicated in your question (TPS61096A) might do but, bear in mind that it runs from a 5 volt rail normally and, has output current restrictions that may reduce the amount of current you might want to deliver to your LEDs.

• I selected it for the number of channels, operational voltage, and max output voltage. I must admit I maybe did not understand the part you highlighted. I thought it meant I could send up to 50mA in the LED outputs. If the driver itself fixes the current to 50mA then I cannot use it, the LEDs will die. (They are low-power, between 2 and 4 mA operating current). – Silver Quettier Apr 25 at 11:09
• Maybe you should take a closer look - let me know if you need help. – Andy aka Apr 25 at 11:14
• Well... Judging by the 9.2 "Typical application" part of the TLC6 spec, it recommends putting a resistor in front of the LED strips. I got it so far: with 6 green LED with a forward voltage of 2.8V, if I tune my boost coverter to provide 20V, I need to put a 1kΩ resistor to use the remaining 4.2V and set the current to 4mA. And I can try to reach better efficiency by lowering the input voltage as much as possible. But what of the change of LED forward voltage with temperature increase? If it drops to 2.6V, the resistor will take the remaining 9.6V and current will be 9mA... Poof. – Silver Quettier Apr 25 at 12:13
• I assume I can mitigate this effect by using a bigger voltage and a bigger resistor, so the change in forward voltage would be, in comparison, less important. But then my power efficiency becomes terrible, which I can't really afford on a small battery. (Space constraints are strong, this is a hobbist miniature circuit.) – Silver Quettier Apr 25 at 12:15
• Yes, it's the age old problem with LEDs, temperature and current limit resistors. I haven't looked deeply into the spec but if you can PWM the drive, then you can use smaller resistance and modulate on-off. You can do this because peak current for any LED is usually much greater than max continuous current. – Andy aka Apr 25 at 12:19

Your boost approach is correct – what you need is a boost converter.

There's special boost converters that deliver constant current; they're often even sold specifically as LED drivers in their own category!

I'd recommend browsing the "Power Management" categories of TI.com, onsemi.com, diodes.com, maximintegrated.com, …

Depending on the architecture of the boost converter, it might be easy or hard to convert an existing voltage-controlling one to a constant current boost converter: instead of measuring the voltage for the feedback loop using a voltage divider from the generated output voltage to ground, just connect boost converter's feedback input to a shunt resistor, i.e instead of

---------\
|
Boost    |---Vout-----+
conv-    |            |
erter    |            R
|fb----------+
|            R
|            |
---------/           —––
–
.


have

---------\
|           //  //  //      //
Boost    |---Vout--->|-->|-->|- ... ->|---\  LEDs
conv-    |                                |
erter    |                                |
|fb------------------------------+
|                                Rsense
|                                |
---------/                               —––
–
.


Notice, however, that the LED drivers have features that you don't get that way (open LED detection, dimmer inputs, etc).

• If I understand, I would be better off trading the TLC6C5912 altogether for another component that also takes care of DC/DC boost and will manage both LED power and PWM driving? – Silver Quettier Apr 25 at 11:11
• that depends – if you need maximum power efficiency, then yes. If you want to use your TLC6... to control the current with any supply voltage > 20V, you can just pick any boost converter with sufficient output power. Ah, I just saw Andy's answer: yes, he's right: for current control, you can use your TLC6..., so your supply voltage only needs to be high enough. – Marcus Müller Apr 25 at 11:25