High Brightness RGB LEDs

Generating multicolor light using high brightness RGB LEDs

The new high power, high brightness RGB LEDs are coming to be used in many different lighting applications, such as general lighting systems, traffic signals, advertising, backlighting, automotive lighting, etc. The demoboard from STMicroelectronics makes RGB LEDs popular because it makes it simple to generate multicolor light, create amazing special lighting effects and it is easy to change the brightness. And as if that was not enough, the long lifetime and small size of LEDs make them the light source of the future.

Colored light illumination

This design solution shows how to drive these types of RGB LEDs, e.g. Golden DRAGON or OSTAR Projection from OSRAM Opto Semiconductors, using two STP04CM596 (or new version STP04CM05) LED drivers with constant current output per channel (up to 500mA). A reference board known as STEVAL-ILL009V1 was developed in order to demonstrate this design concept. Any laptop power adapter can be used to show the capabilties of the LED array in an office environment.

Main features:

can drive 8 LEDs with 350mA or 4 LEDs with 700mA
LED over-temperature protection
- STEVAL-ILL009V3 with STLM20 temperature sensor
- STEVAL-ILL009V4 with NTC resistor
4V@3A DC/DC converter
Color regulation (manual / auto)
SW brightness regulation
6 lighting modes
64 brightness levels for each LED
Input voltage between 8V and 30V

Power block schematic

The principle of this reference board is shown in the block schematic. The microcontroller (8-bit µC ST7FLITE09) sends the DATA through SPI (serial peripheral interface) to the LED drivers and provides color and brightness regulation. Multicolor light generation is provided by a new technique that allows to drive each output independently using PWM software modulation. A DC/DC converter with L4973D3.3 monolithic converter was designed to provide a proper voltage for the RGB LEDs and improve board flexibility, because of it's wide input voltage range (8V to 30V - could be adapted up to 55V). Besides there is also a LED over temperature protection implemented (STLM20 or NTC) to reduce LEDs power and increase significantly the LED lifetime if the LED temperature is reaching specification limits. Different operating modes, like manual adjustment, white light or automatic color changing are implemented in the driver software.