High power LED driver using LTC3780
OK, so I wanted the ultimate light for my bike. So, I went straight for overkill.
- 3 White Seoul P4-U on the handlebars,
- 1 Red Seoul P4-U in the rear under the saddle,
- 3 White Seoul P4-U on the handlebars on the helmet,
- 1 Red Seoul P4-U in the on the back of the helmet.
At full power that's close to 20W of LED in the front. I should be able to see the road.
1 Seoul P4 on the handlebars earns respect from drivers, so it will make for safer cycling.
Battery voltage is more or less fixed between 10V and 15V because of the car horn (bough at scrap yard, much better than airzound, totally deafening, perfect).
The system will have 2 regulators, one on the bike with the main batteries (8-12 AA), and one on the belt with something like 4 AA batteries and a connector to use power from the bike when riding. This doubles as a backup, and a portable headlamp to read a map, go take a leak, etc. So, each regulator will drive 4 LEDs in series.
Current can be set via a pot from 0 to 1 A.
At 1A light output and power draw are massive, and efficiency is very high, this is perfect for going downhill fast.
At 0.2A it still provides lots of light and run time is very long, which is exactly what I need.
A third regulator (optional) will provide 5V for the GPS since its own batteries don't last long enough to make a decent ride (only 2 hours).
Problem is, LED voltage is in the same range as battery voltage so I need a buck-boost regulator. Also, efficiency of drivers found on the web isn't that high, so out comes LTC3780.
The design is slightly complex and expensive because this circuit uses synchronous rectification and current mode regulation.
Who cares. I like overkill.
Since I was doing some 4-layer boards for the FPGA project, and a slightly larger board was at the exact same price, this reg got the full luxury treatment. So, the board is 4 layers.
A ZXCT1051 current probe with a 0.1 Ohm current resistor is used to regulate the LED current.
Hunting for the right MOSFETs was quite difficult, but DigiKey revealed the FDS8878 which looks good and is very cheap.
Features :
- Input voltage : 6V-20V
- Output voltage : 6V-20V
- Output voltage above, below or equal to input voltage (actually the parts are 30V-rated)
- Output current : settable between 0 and 1A with potentiometer (it would probably deliver up to 30W or something)
- Also configurable as a voltage regulator for powering the GPS etc
- Efficiency : 95% at full power (measured)
- Open circuit protection, short circuit protection, etc
- Hi/Lo beam : press a button and it goes to full power, release and it goes back to normal.
- Size : 40mm x 40mm
Apart from a slight error in the datasheet (you have to connect STBYMD and RUN for the regulator to start), there were no problems. I was quite surprised that my first trip into switcherland so was a success so easily.
It is working nicely. It puts out about 600-800 lumens depending on where the LED numbers come from.
A few problems though :
- Carbon pots are unreliable (mine is starting to give up)
- When used with 6 NiMH batteries, setting it to full power draws a lot of current on the batteries, so at the end of their charge, the batteries' voltage drops below the required voltage for the LTC3780 because of their internal resistance. Solution : if the batteries are a bit empty, use a lower power.