Small LED driver circuit - Page 1
Forum section: electronics / beginners

author: anvoice
author_rank: Frequent Contributor
timestamp: July 07, 2016, 08:23:53 am
content: Hello,Trying to make a very simple LED driver circuit for a 3.7V LED (CREE XM-L T6), which consumes about 300mA. The idea is to control from software (arduino) when it lights up: my guess is the best way to do so is with a transistor. However, researching this topic led me to a lot of information about LED driver circuits and the importance of current limitation to the LED, so I'm wondering if I actually need a driver board, or if I can get away with connecting it through a low-voltage transistor directly to a buck converter set up for 3.7V or a little higher to account for the voltage drop across the transistor. Obviously I'd not like the LED to burn out right away, so if that's a no I can look into a driver.Thank you.

author: Ian.M
author_rank: Super Contributor
timestamp: July 07, 2016, 08:40:46 am
content: Datasheetsays Cree XML series has a max If of 3A. At 300mA you have so much safety margin that the driver isn't too critical, but a fixed voltage source is likely to give trouble with temperature changes casing large current changes. If the buck converter has a current limit you can adjust to the desired current it will be fine - just set the OC output voltage to about a volt above the desired current and set the limit to the current you want with a suitable power resistor as a dummy load. I wouldn't want to go over 1.5A without carefully checking the current limit doesn't spike upwards on startup or shutdown and that the ripple doesn't peak over 3A.

author: anvoice
author_rank: Frequent Contributor
timestamp: July 07, 2016, 09:35:31 am
content: Thank you.So wire a resistor in series with the LED to limit current?My buck converter doesn't have an adjustable current, but it's max current is claimed to be 3A, 1.8 continuous. Would that imply it can't go over 3A and I'm safe or not?

author: Ian.M
author_rank: Super Contributor
timestamp: July 07, 2016, 10:06:58 am
content: It doesn't sound like it has effective current limiting although it may have overload protection (i.e. thermal shutdown). If you are going to use current limiting resistor, it needs to drop at least 25% of the LED (or LED string total) Vf to be effective, and even so don't push it higher than 80% or so of the LED's max If rating.N.B. the LED *MUST* be properly heatsinked and using a resistive current limiter only adds to your heat dissipation problem. If you really only want to run the LED at 10% of its max If, a resistor may be a viable option, otherwise I would strongly recommend a proper switched mode constant current LED driver with a logic level shutdown pin that your Arduino could control.

author: danadak
author_rank: Super Contributor
timestamp: July 07, 2016, 10:09:18 am
content: If buck does not specifically state it has a current limit then the3A is just the max recommended allowed current. Attached is thereason you cannot V drive an LED, its a PN junction, and curvedoes not due justice to additional effects of T as it has a strong Tvs I sensitivity as well.Using R will permit current limit, but wastes a lot of power.http://www.homemade-circuits.com/2015/06/cree-xm-l-t6-led-driver-circuit.htmlGo to TI website, there are tons of buck/boost solutions with current limiting,and the tools to simulate not only electricals but thermals for the driver.Regards, Dana.

author: anvoice
author_rank: Frequent Contributor
timestamp: July 07, 2016, 10:40:19 am
content: Ah, got it.Thank you for the help.

author: CJay
author_rank: Super Contributor
timestamp: July 07, 2016, 03:53:38 pm
content: Quote from: danadak on July 07, 2016, 10:09:18 amThat circuit works nicely with a MOSFET too and can be controlled with an IO pin of a microcontroller which, I think, is another requirement the OP had.See:http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/index.php?topic=worklog&p=5and if the OP follows the tutorial they will also find out how to make the necessary calculations.

author: anvoice
author_rank: Frequent Contributor
timestamp: July 08, 2016, 12:11:24 am
content: Great, thanks a lot, I'll read it.

author: dannyf
author_rank: Super Contributor
timestamp: July 08, 2016, 12:36:27 am
content: QuoteYour first decision is if you want simplicity or efficiency. 1w is doable with a linear driver, but also doable with a smps.If you go down the route of a linear driver, a mosfet would work nicely with your mcu.If you go down the route of a smps driver, you can integrate it with your smps. I did this here:https://dannyelectronics.wordpress.com/2016/05/24/repurposing-smps-modules-as-led-drivers/Most of those smps chips of an on / off pin, or EN/shutdown pin that you can pwm with - typically low speed pwm, to change brightness. Or you can use a digital pot controlled by a mcu to do that.

author: CJay
author_rank: Super Contributor
timestamp: July 08, 2016, 07:38:52 am
content: Quote from: anvoice on July 08, 2016, 12:11:24 amI've used it a few times and controlled it with a PWM fade/flash routine in a PIC 16F675 or '683 to control strings of LEDs, I oversized the FETs because I have a large quantity of IRF521 devices left over from a work project. There's no appreciable heat dissipated at half a watt per device.