WARNING! IT'S UNTESTED - I'VE NEVER BUILT THIS BOARD. But I will :)

LED driver for powering Cree XHP35 LED at 1.05A from 3S LiPoly battery to use in underwater flashlight.

It's currently tuned up to work with 9-12.6V input.

Input can be easily changed to 4-25V with slight adjustment of R2 and R3. Increasing voltage over 25V is limited by used capacitors and FETs. You can change it upto ~60V with selecting new Q1-Q5, but PCB rework is also required.

Current is roughly set to 2A with R11, and is fine-tuned to 1.048A with R14&R15. Tuning R14 and R15 values makes it possible to set any current between 0-2A.

This driver is build around LT3791

I've also added Attiny24 to control PWM input and monitor input voltage. It can be ignored and never placed. Firmware will be added later, but the idea is to detect power loss with Q6, go to deep sleep and change working mode on power recovery. C16 gives uC ~4 seconds of life in power down mode, so we can switch power modes with one romote switch. Simulation gave me 10us for uC to go to deep sleep mode and it seems perfect.

You can download gerbers to order this PCB from SeedStudio - it's very cheap and has nice quality.

BOM - only for registered DigiKey users. Total cost is nearly $27 now.

Yo'll get something like this:

Board is 19.5x35mm and it should generate almost no heat - calculated efficiency is >96.5% and power loss is around 0.4W at 13W output.

It's also round version (32mm):

Part List is also included.

You can also take a look at LTSpice IV simulation and efficiency report. Schematic and PCB created with Eagle 7.5. It's not as nice and clean as I wanted it to be, but it should do the job.

Changes required to use XHP70:

It is very easy to modify this driver for XHP70 LED (12V version, at 2.4A). Modifications are simple:

• change R11 to 0.033 Ohm (sized 1206, power rating - 0.5 to 1W). Something like Panasonic ERJ-8BWFR033V. It will limit driver output current to 3A
• change R15 to 100k (0402 size) to fine-tune output current at 2.4A.
• change L1 to 10uH with sauration current at least 4A. Wurth Electronics 7447709100 will fit perfectly
• you can also remove L2 to boost efficiency to >98%, but it requires good wireing and low battery impedance (input current will pulse between 2.7 and 3.5Amps (with spikes to 10-15Amps) for 0.01 Ohm battery impedance). Instead we loose ~0.2W at L2, but have no power pulsation on input (<0.04A & stable input current 3.2A).

LTSpice simulation results:

• input power: 36.3W @ 11.7V
• output power: 35.5W @ 2.42A
• calculated efficiency: 97.8%
• LED current pulses: <1.7%