WHITE LED/OLED STEP-UP CONVERTER
FSP3304
Start-up and Inrush current
In order to facilitate quick startup, a soft-start circuit is not incorporated into the FSP3304. When the IC is first turned
on with no external soft-start circuit, the peak inrush current is about 400mA. Figure 8 shows an implementation for
soft-start. When soft-start and dimming controls are used simultaneously, a low frequency PWM signal (less than
10kHz) or the methods in Figures 5, 6, and 7 should be used.
Open circuit protection (white LEDs)
If one of the LEDs is disconnected, the FB voltage drops to zero and the IC switches at maximum duty cycle. This
results in a high voltage that may exceed the SW voltage rating. To limit this voltage, use a Zener diode as shown in
Figure 9. The Zener voltage must be larger than the total forward voltage of the LEDs and the current rating should be
higher than 0.1 mA.
Board layout
To reduce EMI, minimize the area and path length of all traces connected to SW. Use a ground plane under the
switching regulator and connect R1 directly to the G pin of the IC.
Figure 8: Soft-start circuit
Figure 9: Open-circuit protection
5/9
2007-4-24
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
