led driver question - Page 1
Forum section: electronics / beginners

author: vivi-d
author_rank: Regular Contributor
timestamp: November 12, 2022, 07:41:23 pm
content: Hello, I'm designing an LED ring for my loupe. I came across this part which looks like a good LED driver. If you look at the last page of this datasheet there are two circuits shown. My question is would it be more efficient to use PWM to limit current instead of resistors? Wouldn't the resistors just waste battery power?datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/32005f.pdf

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 12, 2022, 07:55:18 pm
content: Yes, any limiting resistors will waste power. The best is to use constant current switched regulators.What you linked is a constant voltage charge pump, has no way of current control, so more circuitry should be added.There're efficient ways of current regulation, for example with a shunt resistor that measure the current, amplifying it with an op-amp and feed it to the converter feedback pin.That way it'll work in constant current mode, that's what you want for a LED. Using LED series resistors is the worst way of allBut better to get a led driver made specifically for this, there're like a trillion leds drivers in the market of all kind.

author: mariush
author_rank: Super Contributor
timestamp: November 12, 2022, 08:07:53 pm
content: That's not a led driver, it's a charge pump .... it may work as a led driver due to the inherent maximum current capability but it's not a led driver.Digikey, Mouser, Newark/Farnell etc all have led driver IC sections, where you can pick loads of led driver chips.Here's for example digikey's led drivers :https://www.digikey.com/short/j4zphhpqI've filtered the list a bit , hid the AC / offline led drivers as I doubt you're gonna use AC voltage on a loupe, set to show only 1 channel drivers, and drivers that can do 50mA to 10A of current.For a step down driver, you can check out PAM2804 - up to 1A of current :https://www.diodes.com/assets/Datasheets/PAM2804.pdfFor step up driver, check out AP3031 -https://www.digikey.com/en/products/detail/diodes-incorporated/AP3031KTR-G1/4470853- or AP5725 -https://www.digikey.com/en/products/detail/diodes-incorporated/AP5725WG-7/2755851If you plan to power it from 5v or some other low voltage (ex 2 AA batteries or a lithium cell) then you probably want a step-up / boost led driver

author: MrAl
author_rank: Super Contributor
timestamp: November 13, 2022, 06:23:03 pm
content: Quote from: vivi-d on November 12, 2022, 07:41:23 pmThis question comes up now and then because it seems that if the LED turns on and off it will use less power than if it runs constantly.The truth is that PWM by itself does not increase efficiency. The LED is off for some time, but during the 'on' time it must pass more current in order to reach the same brightness level. In doing that it must also pass current though some resistance. Even though the resistance is less than when driven directly, the fact that the current is higher means it's about the same efficiency. In fact, if we look at the efficacy of an LED as the current increases it does down, so the overall efficiency also goes down a bit. It's not a huge difference but it does go down because the LED brightness does not increase in a linear fashion but drops slightly as the current increases.In order to see a real increase in efficiency that can be a lot higher, you have to use PWM in conjunction with an energy storage device such as a capacitor or inductor. Usually an inductor. That's the basis of a buck converter. It uses PWM but with an inductor (and often a filter cap too) and that provides a TRUE power conversion. That means that if you input 10v and output 5v to drive an LED and the LED current is 1 amp, the input will be just 1/2 amp. That means the same power that goes out is absorbed by the input. That's with a 100 percent efficient circuit but most circuits are 80 to 90 percent efficient so there is still some power loss, but it's not nearly as much as direct drive or PWM alone.One advantage PWM has over a regular resistor though is it is easier to provide variable brightness. A PWM signal isnt too hard to vary using a microcontroller so you can get a wide range of brightness levels. To do that with a resistor you would need a power potentiometer which would be big and expensive.If you can though, it is better to use a buck converter and that will provide a wide range of brightness levels as well as decent efficiency.PWM alone does not increase effciency.