ZXSC400_06 Datasheet, PDF (1/4 Pages) Zetex Semiconductors – Photoflash LED

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
L1
U2
VBATT
SW1
U1
V_CC
VDRIVE
STDN ISENSE
GND
V_FB
C1 ^ZXSC400
R3
R4
SW2
C2 ^R2
R1
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
1
© Zetex Semiconductors plc 2006
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