DN74 ZXSC400 Photoflash LED reference design

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.

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