3W HIGH POWER WHITE LED DRIVER

2007-7-13

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FSP3604

FUNCTIONAL BLOCK DIAGRAM

APPLICATION INFORMATION

Inductor Selection

The FSP3604 can use small value inductors due to its switching frequency of 1 MHz. The value of inductor will focus

in the range of 2.2uH to 4.7uH for most FSP3604 applications. In typical high current white LED applications, it is

recommended to use a 4.7uH inductor. The inductor should have low DCR (DC resistance) to minimize the I

power loss, and it requires a current rating of 2A to handle the peak inductor current without saturating.

Capacitor Selection

An input capacitor is required to reduce the input ripple and noise for proper operation of the FSP3604. For good

input decoupling, Low ESR (equivalent series resistance) capacitors should be used at the input. At least 2.2uF

input capacitor is recommended for most applications.

A minimum output capacitor value of 6.8uF is recommended under normal operating conditions, while a 10uF-22uF

capacitor may be required for higher power LED current. A reasonable value of the output capacitor depends on the

LED current. The ESR of the output capacitor is the important parameter to determine the output voltage ripple of

the converter, so low ESR capacitors should be used at the output to reduce the output voltage ripple. The small size

of ceramic capacitors is an excellent choice for FSP3604 applications. The X5R and X7R types are preferred

because they maintain capacitance over wide voltage and temperature ranges.

Diode Selection

It's indispensable to use a Schottky diode rated at 2A with the FSP3604. Using a Schottky diode with a lower forward

voltage drop is better to improve the power LED efficiency, and its voltage rating should be greater than the output

voltage.

In application, the ON semiconductor MBRA210LT3G is recommended.

LED Current Setting

The LED current is set by the single external Rs resistor connected to the FB pin as shown in the typical application

circuit on page 1. The typical FB reference is internally regulated to 95mV. The LED current is 95mV/Rs. It's

recommended to use a 1% or better precision resistor for the better LED current accuracy. The formula and table 1

for Rs selection are shown as follows:

Rs=95mV/I

Table 1. Rs Resistor Value Selection