If the OP wants to treat them as *one* lamp, I would seriously consider a small DC-DC converter to kick the 5V (assuming it wants to be entirely powered off the USB) supply up to "whatever" @ 10mA. Note that you could design the supply so that *it* does the intensity control (current mode output that tracks something from the MCU).
I would be more concerned about having enough Icc from the USB port to deliver what the lamps will consume. (high efficiency lamps?)
Welcome to USENET!
Would work, but I'd be concerned with process variations between the LEDs leading to the pair with the lowest forward voltage drop hogging all of the current. Less of a problem (and likely a non-problem) if they're all from the same batch, of course.
A current mirror is probably the best approach for balanced control. Also opens the possibility of controlling the brightness by controlling the current directly (and avoiding flicker or beat frequencies).
Have something to add? Share your thoughts — no account required.
Ask the community — no account required