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October 24, 2009
Open-source hardware developer from Michigan
about a year ago
If you want to drive non-addressable LED strips, I’d recommend PWMing some mosfets. Current-limiting resistors are built into the strips, so you don’t need a constant current power supply.
I think that should work fine. Considering the efficiency of the regulator and on-resistance of the switching FET (typically about 0.25 ohms) I don’t think there should be much of an issue. the AL8805 should be able to regulate with ~1V of headroom, although I haven’t personally tested it.
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about 2 years ago
The upcharge also includes a bit of markup–the reg B boards were sold with zero profit, so there was trouble ramping up the supply chain (hence the thousands of backordered boards). Adding some profit in should allow them to scale to meet demand.
about 2 years ago
I haven’t driven a PicoBuck with a DAC, but I have tested analog control with a bench power supply as the control input. Just have your DAC generate a voltage between .5V-2.5V to the control pins. The control pin only takes microamps to drive, so you shouldn’t need any buffering of your DAC’s output. Keep in mind that analog control can only dim from ~20% to 100%, so you may still want PWM if you need low (<20%) brightness.
If you haven’t already, contact firstname.lastname@example.org and let them know that your PicoBuck failed when operating within the product specs. PicoBucks have been run above 19v successfully, so you probably just got a bad board.
I’m unfamiliar with the WS2801, but it does look like you could use that IC to drive the inputs of the PicoBuck. The datasheet includes an app note where the WS2801 is driving external NPN transistors–all you’re doing is replacing that transistor with a channel of the PicoBuck
There are 2 ways you can adjust the brightness: apply a 0-2.5v analog voltage to each of the control pins, or send a PWM signal to each input. Check out the “Hookup Guide” linked above, it should point you in the right direction.
The PicoBuck should be able to handle around 700mA of current if you replace the current limiting resistors with 0.14Ω. I wouldn’t go much above 700mA, as that is approaching the current limit of the diode. Also, if you don’t need independent control of each LED, you can attach multiple LEDs of a color in series (as long as your input voltage is 1.5v+ more than sum of your forward voltages).
A large difference between Vin and Vfwd (running a 2.5v led at 700ma from a 9v supply ) yields about 76% of efficiency. Increase that to a Vfwd=5v and efficiency jumps to 88%. In either case, you shouldn’t need any heatsinking (as far as the numbers go).
The current drive of the PicoBuck is 350mA per channel (set by an 0805 current-sensing resistor). My site (protofusion.org) should be back up this evening! We just had a hardware failure.
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