DN74 ZXSC400 Photoflash LED reference design Description This design note shows the ZXSC400 driving a Photoflash LED. The input voltage is 3V with a maximum pulsed output current of 1A for 2ms. A typical schematic diagram is shown in Figure 1. C3 U2 L1 SW1 VBATT U1 VCC VDRIVE STDN ISENSE GND C1 SW2 C2 R2 VFB ZXSC400 R3 R1 R4 Charging mode: SW1 closed, SW2 open Discharging mode: SW1 open, SW2 closed Figure 1 Schematic diagram Operation In charging mode, SW1 is closed and SW2 is open the ZXSC400 is configured as a typical boost converter, charging capacitor C2 up the regulated output voltage set by the ratio of R1 and R2. This is typically 16V. The peak current of the converter (current drawn from the battery) is controlled by R3 plus R4, and is typically 280mA for this application. When C2 is charged to 16V the SW1 is opened and SW2 is closed, converting the ZXSC400 to a step down converter to provide a 1A constant current for 2ms to the photoflash LED. During step down operation, current flows from C2, through the photoflash LED, L1, U2 and is returned to C2 through R3. This means that the peak current is set at a higher value than in charging mode, typically 1A. When the current reaches it's peak value, U2 is switched off and current flows from L1 through the Schottky diode in U2, to the photoflash LED. This cyclic process is repeated until C2 is discharged. Issue 2 - July 2006 © Zetex Semiconductors plc 2006 1 www.zetex.com DN74 Ref Value Part number Manufacturer Comments U1 ZXSC400E6 Zetex LED Driver in SOT23-6 U2 ZX3CDBS1M832 Zetex Dual NPN and Schottky L1 12H Generic Generic ISAT=1A R1 10k⍀ Generic Generic 0805 size R2 510k⍀ Generic Generic 0805 size R3 22m⍀ Generic Generic 0805 size R4 100m⍀ Generic Generic 0805 size C1 1F Generic Generic C2 150F Generic Generic C3 1F Generic Generic Table 1 www.zetex.com Bill of materials 2 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 DN74 Typical operating waveforms (For typical application circuit where VBATT = 3V and Tamb = 25°C unless otherwise stated) Figure 2 Issue 2 - July 2006 © Zetex Semiconductors plc 2006 Performance graphs 3 www.zetex.com DN74 Intentionally left blank Europe Americas Asia Pacific Corporate Headquarters Zetex GmbH Streitfeldstraße 19 D-81673 München Germany Zetex Inc 700 Veterans Memorial Highway Hauppauge, NY 11788 USA Zetex (Asia Ltd) 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 [email protected] Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 [email protected] Telephone: (852) 26100 611 Fax: (852) 24250 494 [email protected] Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 [email protected] For international sales offices visit www.zetex.com/offices Zetex products are distributed worldwide. For details, see www.zetex.com/salesnetwork This publication is issued to provide outline information only which (unless agreed by the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contact or be regarded as a representation relating to the products or services concerned. The company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service. www.zetex.com 4 Issue 2 - July 2006 © Zetex Semiconductors plc 2006