Linear LED driver... can I just PWM the whole thing?  - Page 1
Forum section: electronics / beginners

author: doublec4
author_rank: Regular Contributor
timestamp: April 22, 2021, 05:53:30 pm
content: I've been looking at linear LED drivers as I need to fit a circuit in a very small space (like an LED light head)The system will run off of a battery, from about 12.4V down to 9.3V and I plan on making light heads with single colors ie. white / amber / red / blue and some dual color ie. amber / red in one package... 150mA should be plenty bright to for a current to push through some LEDs for my application.Therefore component count and component size is important as I previously stated.I would like to connect these light heads to another control board I am also designing. The control board (using ATMEGA uC) could ideally flash the LED light heads on / off in patterns, as well as PWM them to dim the lights. This where I am unsure how to approach this. Ideally I would like to just have power and gnd wire coming off of a light head and I could use a N channel mosfet on my control board to low side switch the light head GND wire to flash / dim the LED light head. In the case of a dual color light head, I could PWM the GND wire for each of the different colors to dim/flash/turn them off.However, when looking at some of the linear led drivers, ex:https://www.diodes.com/assets/Datasheets/AL8400.pdfhttps://www.diodes.com/assets/Datasheets/AL5812.pdfThere is no mention of PWM in AL8400, and for AL5812 they show the n channel mosfet location for PWM between the Rset and GND for that pin. In the case of my application, I guess I would put the n channel mosfet on my control board, and this would add an additional wire to go from the Rset to the drain of the mosfet. So a light head with a single color would have power / gnd / control wire. For dual color heads there would be power / gnd / and a control wire for each color.So perhaps my question is silly, but would it cause any harm to just use an n channel mosfet and low side switch the entire GND of the entire light head with either AL8400 or AL5812? If the PWM frequency is not very fast, ex 250Hz is there some risk of damaging the linear driver by turning the whole thing on and off? Would it cause poor current regulation of the LEDs? I would imagine there is some sort of time involved for the device to turn on and settle and regulate current accurately...? Doing it this way a single color light head would only have power and gnd wire, and a dual color would have power and then 2 gnd wires, one for each color. What is the issue doing it this way?Hope my question is clear! Thank you!

author: Manul
author_rank: Super Contributor
timestamp: April 22, 2021, 09:40:38 pm
content: Maybe, but it is not how it is supposed to be working, so you are on your own. It is off-map situation, because datasheet does does not provide the data. A lot of things to consider, like input capacitance, how fast does it regulate, will it overshoot, etc.PWM is very well suited for inductive loads. It is also good for resistive loads. But for loads which are fully or partly capacitive it is kind of (for the lack of better word) pornographic. First, you get huge current spikes at turn on. Also, every time this capacitance is charged up, there is 50% energy loss associated with that charging. These losses might be significant. Second, if capacitance is large or frequency is high it is not anymore a duty cycle control. It becomes more like voltage control, a lousy PWM to voltage converter.If the linear regulator needs some input capacitance (for stability), then it is already one problem. Also other things might be a problem, like regulation, because PWM will effectively create huge input voltage transients.

author: doublec4
author_rank: Regular Contributor
timestamp: April 22, 2021, 10:47:19 pm
content: Thank you! So I'll probably just use the second option and go with the dedicated PWM wire. It's not a huge deal but if I could avoid it I would have... doesn't sound ideal to do it that way though...