Issue 2 - July 2006

1

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© Zetex Semiconductors plc 2006

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.

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.

GND

STDN

U1

ZXSC400

C2

L1

VBATT

C1

R3

R1

R4

R2

C3

SW2

SW1

Charging mode: SW1 closed, SW2 open

Discharging mode: SW1 open, SW2 closed

U2