© 2011 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5346 • Rev. 1.0.1

11

Application Information

The reference schematic diagram is shown in Figure 29.

FAN5346 is able to drive up to eight LEDs with input voltage

number of LEDs that can be used FAN5346 depends on

forward voltage. It is recommended that the forward voltage

maximum LED current be 20mA. FAN5345 can be also used

directly. The return trace of the load should also return to

GND through a sense resistor (R1).

Figure 29. Reference Application Schematic Diagram

Component Placement and PCB Recommendations

FAN5346 switches at 1.2MHz to boost the output voltage.

Component placement and PCB layout need to be carefully

taken into consideration to ensure stable output and to

prevent generation of noise. Figure 30 is a portion of the

evaluation board layout. The critical layout elements are: the

Figure 30. Reference PCB Layout

Input Capacitor and Return Trace

The input capacitor is the first priority in a switching buck or

enables a switching regulator to deliver its best performance.

During the regulator’s operation, it is switching at a high

since it is trying to make the input source vary at the same

switching frequency as the regulator. To ensure a stable

the input pin of the regulator and the return trace from GND

possible to minimize trace resistance, inductance, and

high-frequency variation due to switching. Trace resistance

reduces the overall efficiency due to I

trace inductance could effectively yield ground variation to

add noise on VOUT. The input capacitor should be placed

close to the VIN and GND pins of the regulator and traces

should be as short as possible. Avoid routing the return trace

through different layers because vias have strong inductance

effect at high frequencies. If routing to other PCB layers is

unavoidable, place vias next to the VIN and GND pins of the

regulator to minimize the trace distance.

Output Capacitor and Return Trace

The output capacitor serves the same purpose as the input

capacitor, but also maintains a stable output voltage. As

explained above, the current travels to the load and back to