The CL8800 sequential linear LED driver IC from Microchip may be used to drive a long string of inexpensive, low-current LEDs directly from the AC mains. A linear LED design based on the CL8800, such as a fluorescent tube replacement, can achieve high efficiency and power factor without a requirement for external magnetics or capacitors.
This is because the CL8800 can step down a mains voltage without using conventional inductor-based switching techniques. To do so, a long string of LEDs is tapped at multiple locations, each tap having a linear current regulator that controls the LED current of upstream string segments. The output current at each tap is adjustable individually by selection of a resistor.
As the instantaneous rectified input voltage rises, downstream segments of LEDs receive enough voltage to begin conducting. Once a segment starts conducting, the downstream regulator takes over from the upstream regulator. This limits the voltage across the regulators when they are conducting, thus keeping power losses low.
A basic driver circuit consists of the CL8800, six resistors and a bridge rectifier. Two to four additional components may be added to provide various transient protection. No capacitors, EMI filters or PFC circuits are needed.
In larger systems, cross-regulation as the CL8800 switches from one regulator to another, makes for smooth transitions.
The current waveform can be tailored to optimise for input-voltage range, line/load regulation, output power/current, efficiency, power factor, harmonic distortion, dimmer compatibility and LED usage.
With the addition of an RC network, the driver supports phase dimming.
- Fluorescent tube retrofits
- Incandescent and compact fluorescent bulb replacements
- General LED lighting
- Up to 7.5W output (13W with a heatsink)
- 85% typical electrical efficiency
- >0.95 power factor
- <20% total harmonic distortion of the line current
- Low levels of conducted EMI without the need for filters