LED Driver Circuit Question

I’m attempting to drive Cree XLamp XP-G2 High Power 3-up LEDs ( Cree XLamp XP-G2 LEDs (ledsupply.com)) with these drivers: RCD-24-0.70/W/X1 Recom Power | Power Supplies - External/Internal (Off-Board) | DigiKey and power them via 5v battery packs from Voltaic Systems (USB Battery Pack | 19,200mAh 71Wh (voltaicsystems DOT com), and controlling the dim feature of the driver via a PWM pin on an Arduino. This isn’t working, can you help me solve the issue? Using a bench top variable power supply I find that the driver won’t light the LED with less than 6.5v minimum. I’m also finding that the PWM pin connected to the dimmer pin of the driver won’t work. Can you point out my errors?


It appears that the supplier of your arrays reports per-die electrical characteristics, and assumes folks will recognize that a 3-series array will require 3x the voltage to drive it compared to a single-die assembly. The mentioned converter is of a buck topology and can only provide output voltages less than the input, so it’s entirely reasonable that you’d see little or nothing out of it with a 5V source. I’d plan on providing 9 at a minimum to get full output when driving the mentioned LED array, and it’s reasonable for the dimming function not to behave properly when the input voltage isn’t sufficient.

On the dimming front though, the X1 suffix would appear to call out an analog dimming input, though you mention PWM. Unless you’re filtering that PWM output, you might get some odd behaviors when applying it to a dimming input anticipating an analog signal.

Thank you Rick_1976. I can change power sources and provide 9v, but it does sound like a different driver, one with boost rather than buck capacities and digital PWM dimming specs would be better for my purposes. Can you recommend a better driver?

I don’t see any listings for 5V compatible packaged boost drivers that’ll output anything close to 700mA. It’s a somewhat sketchy topology that has ambitions of becoming a short circuit…

A higher voltage source is probably the best answer if it’s a possibility. You could also re-configure that emitter to operate as a parallel array, though it’d be a bit of of a kludge and without changing drivers you’d have little benefit over using a single-die model.

Hi studio,

If you could get by with only 350mA, the 4015-D-E-350 might be worth a look. It’s stretching its capability, as its minimum input voltage is 5V, the output voltage must be at least 3V above the input voltage, and if doing digital PWM, the max frequency is only 1KHz. Here’s the link to the datasheet, and here’s a wire harness, the 3021HE, designed to work with it.

It is primarily designed for analog dimming with a potentiometer, but one might be able to do analog dimming via PWM fed into a good low-pass filter or with a D-A converter, such as the 1528-1010-ND.

One other thought: If this is a one-off application, you could also sort of Rube Goldberg something together with a DC-DC boost converter and then follow that with a more standard buck-style LED driver. It would lose efficiency with the additional conversion step, and take a lot more space, but it would probably work.

You could use something like the DFR0123 adjustable boost supply along with any number of more typical buck LED drivers behind that.

Hi thank you David_1528 and rick_1976, this really helps and I think I could do either of those options, however if I switch to a 9v power source there will likely be a better driver with digital dimming that will work better with Arduino without the need of a DAC, yes? Could you recommend an inexpensive driver for this?

Hi Studio,

In any buck-type LED driver, you’ll need a little headroom between your input voltage and your output voltage to get proper control. This means you’ll need to be at least 2V (and maybe 3V) above your output voltage (depending on specifics of the particular driver and the amount of current it is sourcing), so, since your LED module will be anywhere from 8.4V to over 10V, you probably ought to have a supply voltage of at least 12V for a buck driver.

Edit: One exception I just found to this is the LDB-600LW, which is a buck-boost topology with a minimum input voltage of 9V. Primary limitation of this part is that its max output current is only 600mA.