WHITE LED/OLED STEP-UP CONVERTER

FSP3304

2007-4-24

5/9

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