Is this common?  Boost regulator driving LED in this way? - Page 1
Forum section: electronics / projects

author: passedpawn
author_rank: Contributor
timestamp: February 14, 2024, 04:42:51 am
content: A friend suggested this ckt to drive LED at a specific current. Creates a current regulator out of the boost regulator. After many years of doing this differently, was a bit of a DUH moment for me.

author: Andy Chee
author_rank: Super Contributor
timestamp: February 14, 2024, 04:52:59 am
content: In principle, it's similar to the constant current linear regulator arrangement, which I've used plenty of times with the LM317 for powering laser diodes.

author: inse
author_rank: Super Contributor
timestamp: February 14, 2024, 05:00:16 am
content: What is the specific issue you have with the circuit?If your supply is below the LED voltage you need a booster, if it’s above, a resistor, the LM317 circuit or a buck converter come into play.It definitely makes sense to directly regulate the current with the controller so you don’t lose too much energy in the series resistor.There are even specific current feedback controllers with a lower feedback voltage, increasing efficiency even more.

author: Berni
author_rank: Super Contributor
timestamp: February 14, 2024, 06:47:56 am
content: It is perfectly normal to abuse the FB pin like that.There is nothing inherently special about purpose made LED driver switchmode chips. When the chip sees the FB pin being too low, it calls for 'moar powa' by increasing the PWM duty cycle. The difference is mostly that the FB voltage tends to be lower so that the shunt resistor can have a lower voltage across it and so not get so hot.I have done plenty of FB pin abuse before, adding things like over voltage protections, current limits, even inverting the FB with an opamp so that i could use an inverting buck boost topology.

author: ArdWar
author_rank: Frequent Contributor
timestamp: February 14, 2024, 07:41:13 am
content: Not only that this is very common, this isn't even "abuse" per se.Just beware not to overdo your C2 (in your schematic).

author: prosper
author_rank: Regular Contributor
timestamp: February 15, 2024, 07:56:49 pm
content: super common. There are driver IC's optimized for this operation, too (i.e. low FB voltage). Seehttps://datasheet.lcsc.com/lcsc/1811131826_XLSEMI-XL6013E1_C73019.pdfschematic on pp7

author: prosper
author_rank: Regular Contributor
timestamp: February 15, 2024, 08:00:11 pm
content: Quote from: Berni on February 14, 2024, 06:47:56 amIt's also a handy place to apply a PWM (or DC) signal for dimming a LED

author: passedpawn
author_rank: Contributor
timestamp: February 16, 2024, 01:08:09 am
content: thanks guys. i guess i already knew(?) the answer, but was looking for confirmation. i've been doing this in a less efficient manner for so many years. as i said, "duh"

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 16, 2024, 01:23:01 am
content: Yes, the benefit is it will step up the voltage just enough to get Vfd(LED) + Vfb. So for best effiency, this simple scheme benefits from boost converters with the lowest Vfb possible, since it will dissipate I(LED)*Vfb in R4.