FAN5346 查看數據表（PDF） - Fairchild Semiconductor

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FAN5346

Series Boost LED Driver with PWM Dimming Interface

Fairchild Semiconductor 

Application Information

The reference schematic diagram is shown in Figure 29.

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

equal to or greater than 2.9V (VIN ≥ 2.9V). However, the

number of LEDs that can be used FAN5346 depends on

forward voltage. It is recommended that the forward voltage

(VF) of the white LED be no greater than 3.2V and the

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

as a boost convertor by connect the VOUT point to the load

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

L1, CIN, CIN return trace, COUT, and the COUT return trace.

Input Capacitor and Return Trace

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

boost regulator PCB layout. A stable input source (VIN)

enables a switching regulator to deliver its best performance.

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

frequency, which makes the load of CIN change dynamically

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

switching frequency as the regulator. To ensure a stable

input source, CIN needs to hold enough energy to minimize

the variation at the input pin of the regulator. For CIN to have

a fast response of charge / discharge, the trace from CIN to

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

of the regulator to CIN should be as short and wide as

possible to minimize trace resistance, inductance, and

capacitance. During operation, the current flow from CIN

through the regulator to the load and back to CIN contains

high-frequency variation due to switching. Trace resistance

reduces the overall efficiency due to I2R loss. Even a small

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.

Figure 30. Reference PCB Layout

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

the COUT GND terminal. COUT should be placed close to the

VOUT pin. The traces of COUT to L1, VOUT, and the return

© 2011 Fairchild Semiconductor Corporation

FAN5346 • Rev. 1.0.1

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www.fairchildsemi.com

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