Compact but efficient LED driver with dimming - Page 1
Forum section: electronics / projects

author: pechvoegel
author_rank: Newbie
timestamp: January 26, 2021, 05:11:12 am
content: As part of a personal project I want to independently drive a red, a green, and a blue LED at up to 700mA off one or two lithium ion cells, with reasonable efficiency (~80% would be good), at least 100:1 dimming, and it needs to be as compact as possible. Would anyone have any recommendations on how to achieve this?The issue with a normal current controlled buck converter is the size of the inductor. I just don't think have the space for three ~47uH inductors.TheLTC3215looked promising, but it doesn't look like it is designed to work with PWM dimming.A linear regulator should work fine for the red LED because it's forward voltage is significantly below that of one lithium ion cell; but the blue and green both have forward voltages about the same as one cell, so I would have to go to two cells and then get <50% efficiency.

author: Nominal Animal
author_rank: Super Contributor
timestamp: January 26, 2021, 06:04:15 am
content: How about threeDiodes Inc. AL8805s, as used in SparkFunFemtoBuckLED drivers (schematic PDF)? It does need at least 6V input, so you'd need two Li-ion cells in series; and the inductors are 33µF.Another option isTI TPS61169, which works for 2.5V to 5.5V input voltages only (so a single Li-ion cell), needs only a couple of ceramic capacitors and a 4.7µF inductor, and a sense resistor (R = 0.204  / I , so about 0.291 Ohm for 700mA), but I'm not sure of its efficiency with just one LED (as opposed to multiple LEDs in series, all driven at the same current). Also, it requires a PWM dimming signal (5 kHz - 100 kHz).I have played at designing a tiny (12.7mm × 12.7mm)TI TPS61169 + ATtiny10board, but I'm just an uncle bumblefuck hobbyist, and haven't built it yet to see if it works. You're better off following the TI datasheet! But it might give you ideas.

author: james_s
author_rank: Super Contributor
timestamp: January 26, 2021, 06:55:51 am
content: How small does it need to be? I've used the PT4115 before at similar power levels, it can work with an inductor about 8mm diameter.

author: pechvoegel
author_rank: Newbie
timestamp: January 26, 2021, 11:41:55 am
content: Thank you for the suggestions.QuoteThe inductors used there are 6mm across, which is good, but I'm still going to struggle to fit three of those in with everything else.QuoteIt doesn't look like it is intended to work in situations where the input voltage will often be about equal to or a little higher than the output (led) voltage. It could end up being easier to just put a second set of LEDs in series so that this will definitely work. The other option that I'm considering at the moment is making my own 1.33x charge pump.

author: pechvoegel
author_rank: Newbie
timestamp: January 26, 2021, 11:47:40 am
content: QuoteThanks for the part reference. I'll keep it in mind.I'd love to be able to get the RGB LED and the driving circuitry on a little PCB of 20mm diameter, but I can go a bit larger if I absolutely have to, and smaller would be better. The LED would have to be in the center, so that makes placing other components harder.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 30, 2021, 07:57:49 pm
content: Quote from: pechvoegel on January 26, 2021, 11:47:40 amIf you can't fit the drivers on a single board, you can use a stack of 4 boards.