The reference design, with Texas Instruments' LM3431 3-channel, constant-current LED driver, uses two strings of six series-connected LEDs at a current of 30 mA per string.  Each LED has a forward voltage drop of 3.5V to 4.5V, making the total voltage across each string between 21V and 27V. With a 12V power source, the LM3431 is configured as a boost converter to generate this 21V to 27V output voltage.  The design also includes a circuit utilizing a photodiode to make the brightness of the LEDs proportional to ambient light, improving the readability of the display under all lighting conditions.