STEVAL-ILL017V1 STMicroelectronics

AN2811 Application note 3.5 W non-isolated offline constant-current LED driver based on VIPER17 Introduction High brightness LEDs are becoming a prominent source of lighting. Compared to conventional incandescent bulbs, high brightness LEDs (light emitting diodes) have advantages in higher light efficacy, much longer life and faster reaction time in a smaller profile. Since LEDs cannot sustain high voltage stress directly from an AC source, providing a reliable constant-current source to drive LEDs becomes fundamental. This solution provides even luminosity, reliability, the highest efficacy and the longest operating life for LEDs. This application note describes the non-isolated offline constant-current driver based on the VIPER17HN (high frequency version). This solution operates with an AC line input range from 176 V to 264 VAC and provides 500 mA constant current from a 7 VDC source. It can illuminate two LEDs in series. This device is an offline converter with an 800 V rugged power section, a PWM control, twice the level of overcurrent protection, overvoltage and overload protections, hysteretic thermal protection, soft-start and also safe auto-restart after any fault condition removal. The embedded brownout function protects this switch mode power supply in case the main input voltage falls below the specified minimum level for this system. Figure 1. STEVAL-ILL017V1 demonstration board - V June 2009 Doc ID 14904 Rev 1 1/25 www.st.com Contents AN2811 Contents 1 Safety instructions 5 2 Design considerations . 6 2.1 Selected topology 6 3 General circuit description 8 3.1 Schematic diagram . 8 3.2 Bill of material . 9 3.3 PCB layout view 11 3.4 Transformer design 11 4 Test results and waveforms 13 5 Connection of AC line and LED lamp to the demonstration board 21 6 Conclusion 22 7 References 23 8 Revision history . 24 2/25 Doc ID 14904 Rev 1