ledled-driver
I'm planning on building a low-cost LED fixture and I'm a bit confused with how I should drive it. So, if we consider these 2 led cobs:
If I drive them both with a 300mA constant current driver, I have the following questions:
I have controlled the high current LEDs with both current sources and current sinks.
I am just finishing up the testing on an LT3755 driver circuit (current source) that is driving 12 3W Cree XLamps. This driver has excellent performance if you are careful with the layout. This device can do almost a 10:1 boost at 50W! Being able to simulate in LTSpice was extremely convenient.
If you want a simpler layout you could look at current sinks. You would need to get a supply capable of producing a voltage slightly greater than the sum of the Vf(max) of your LEDs. Using a FET and op-amp you would create a voltage controlled current sink. You could PWM this by setting the current to zero. I use an analog switch on my current-sink.
Here is a picture of my current sink design -- http://wiblocks.luciani.org/not-quite-ready/ZB1-ISINK-index.html The FETs are rated for 40A, the connectors are rated for 10A. For an LED application with a reasonable size heatsink you can probably go to 2A. The LEDs in the pictures are being driven at 350mA. No heatsinking is required if the dissipation in the FETs is kept below 1W.
The board in the picture is being controlled by my Arduino compatible NB1 board.
If you power off 9V then consider using some of the 3W LEDs in series: -
Typically red leds need about 2.5V across them so wiring three in series will need at least 7.5V then there should be some current limiting circuit that might take 0.5V.
Typically for blue or green leds they need 3.4V across them so maybe only series together two LEDs.
You might just about get enough brightness with the NCP5623 chip this way but if not you'll need a current limit circuit and mosfets. The NTD80N02 looks OK but you need to limit current into the LEDs.
Best Answer
It's impossible to know the behaviour at a current that is not the specified operating point if there's no datasheet containing an "intensity over I_f" graph – if this is of concern to you, buy LEDs that come with such a datasheet.
Yet another thing I'd like to point out that you don't say anything about your 300mA supply – it might look that an LED that is more efficient at 300mA would be the power-efficient choice, but if it happens to have a lower forward voltage at that current (is there at least an I/U graph?), you'll just "burn" that additional energy (voltage = energy per charge) in a linear regulator.
Now, it's really impossible to tell what kind of regulator you have picked there – yes, this looks a bit like the mains voltage gets rectified on the input side, and then chopped up and downconverted by the transformer type thingie there, but I can't tell the least about how exact the current regulation is (but I do have a hunch that without any secondary side sensing, it's not going to be that exact) or how efficient the conversion. So: if you're at all concerned about operational specifics of your LED system, this is not the supply of your choice.
Generally, people will sell their LEDs at the point at which they produce the maximal amount of light without thermal damage (or without exhaustion of recombination opportunities in the semiconductor), and that will be pretty close to the point of maximum efficiency. So, if you want to drive an LED specified for 700mA at 3.3V with less than its maximum power, you typically use PWM to switch it off and on with a configurable duty cycle, driving it at an "efficient" 700mA when on.