AN2531 Application note Generating multicolor light using RGB LEDs Introduction The new high power and brightness RGB LEDs are going to be used in many different lighting applications as backlighting, general lighting systems, traffic signals, automotive lighting, advertising signs, etc. They are becoming popular mainly because it is possible to generate an easy multicolor light with special lighting effects and their brightness can be easy changed. On top of this, their long lifetime and small size make them the light source of the future. This document describes how to drive RGB LEDs, how to calculate a power dissipation, how to design an over temperature protection, how to use a software PWM modulation and why over voltage protection should be implemented for this kind of application. STEVAL-ILL009V1 reference board shown in Figure 1 was developed in order to demonstrate this design concept. This board was designed for driving super high brightness multicolor RGB LEDs with current up to 700 mA per LED. The LED brightness and color can be very easy changed by potentiometers and an automatic color change mode continuously modulates the color of the LED to generate multicolor light. The LED over temperature protection is designed on this board and therefore the power delivered to the LED can be automatically limited to prevent LED overheating. The STEVAL-ILL009V1 is a mother board assembled without LEDs. To evaluate light effect features, it is necessary to order a load board (additional board with assembled RGB LEDs). Two load boards are available for easy performance evaluation. The first one with the OSTAR Projection Module (refer to Chapter 12, point 1) has ordering code STEVAL- ILL009V3 and the second one with the Golden DRAGON LEDs (refer to Chapter 12, point 2) has ordering code STEVAL-ILL009V4. All technical information about these reference boards such as bill of materials, schematics, software, temperature protection and so on are described in the sections below. Note: A new reference board STEVAL-ILL009V5 was designed in order to replace the former STEVAL-ILL009V1. The main reason why the new board was developed is to demonstrate a new DC/DC converter capabilities using the ST1S10 and new improved LED drivers STP04CM05 and STP08CP05. Thanks to the ST1S10 the size of the inductor is extremely decreased, efficiency improved and board size significantly reduced. The STEVAL- ILL009V5 reference design is described in Chapter 9. Figure 1. STEVAL-ILL009V1 reference board September 2008 Rev 2 1/40 www.st.comContents AN2531 Contents 1 Driving concept for RGB LEDs 6 2 How to drive many LEDs . 7 3 How to set high current for LEDs 9 4 Color control - software modulation . 10 5 Power dissipation 12 6 Over voltage protection . 14 6.1 Description 14 6.2 Type of solutions 15 7 LED temperature protection 16 8 STEVAL-ILL009V1 reference board 17 8.1 General description . 17 8.2 Getting started . 18 8.3 Schematic description . 19 8.4 Bill of material 22 8.5 Design calculation 23 8.5.1 LED supply voltage 23 8.5.2 Temperature protection . 23 8.5.3 SW PWM frequency calculation 25 8.6 Software 26 9 STEVAL-ILL009V5 reference board 30 9.1 STEVAL-ILL009V5 schematic diagram 31 9.2 Bill of material 32 10 STEVAL-ILL009V3 load board 33 10.1 Schematic description . 33 10.2 Bill of material 34 2/40