RGB LED driver for an optical diagnostic instrument. Color accuracy has to be repeatable across units and temperature because the diagnostic algorithm depends on consistent illumination wavelengths.
The Solution
Pure analog current regulation, no PWM. PWM would cause flicker artifacts in the optical measurement. Each RGB channel has its own precision current source with temperature-compensated feedback. The linear current sources are fed by a switching pre-regulator that tracks just above the LED forward voltage, keeping power dissipation low while maintaining clean, ripple-free drive.
Results
Channel-to-channel current matching within 0.5%. The client calibrates each unit once at the factory and the readings hold across the operating temperature range.
How the drive stays clean
Efficiency from the switcher, precision from the linear stage. The switching pre-regulator sits just above the LED forward voltage so the linear sources waste little power, while each channel’s temperature-compensated current source gives the steady, flicker-free light the optical measurement needs.Same light from every unit at every temperature. LED output falls as the junction warms, and red falls fastest, so an uncompensated driver shifts the colour balance as the instrument warms up. Temperature-compensated current sources hold each channel flat across the operating range, and the precision references keep channel currents within 0.5% from unit to unit, so a single factory calibration holds.
Have a Similar Challenge?
Analog LED drive with precision current control is part of the practice.
