SDG #051 Driving RGB LEDs with 1 Constant Current Driver

RGB LED strips and modules look like they should need three independent drivers, one per colour channel, because red, green and blue dies have very different forward voltages and efficiency curves. In practice most low cost commercial strips get away with a single constant current buck regulator, and this video unpicks how that trick works and what compromises it forces on the design.

The key idea is that the buck regulator only has to hold one job: keep a fixed current flowing through whichever LED path is active. Channel selection happens downstream, with switching elements steering that regulated current into the red, green or blue string, or rapidly alternating between them so the perceived colour is a duty cycle blend rather than a genuine simultaneous mix. Because the regulator is a current source rather than a voltage source, it automatically accommodates the differing forward voltage drops of each colour, adjusting its output voltage as the load changes instead of needing per channel ballast resistors.

That has practical consequences worth knowing before you design with or repair one of these strips. Total brightness is shared rather than additive, so white output can be dimmer than the sum of the individual channels suggests. Timing matters too: if the multiplexing rate is low, camera flicker and visible strobing appear, which is why some cheap strips look terrible on video. Understanding the topology also makes fault finding much faster, since a single dead regulator kills all colours while a failed steering device usually leaves one channel permanently on or permanently dark.