FSP3129SD datasheet(4/10 Pages) FOSLINK

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WIDE INPUT 2A STEP-DOWN CONVERTER

2007-1-4

4/10

FSP3129

TYPICAL APPLICATION CIRCUIT

APPLICATION INFORMATION

Output Voltage Setting

Figure 1 shows the connections for setting the output voltage. Select the proper ratio of the two feedback resistors

RFB1 and RFB2 based on the output voltage. Typically, use RFB2≈10KΩ and determine RFB1 from the output voltage:

Figure 1 Output Voltage Setting

Inductor Selection

The inductor maintains a continuous current to the output load. This inductor current has a ripple that is dependent on

the inductance value: higher inductance reduces the peak-to-peak ripple current. The trade off for high inductance

value is the increase in inductor core size and series resistance, and the reduction in current handling capability. In

general, select an inductance value L based on ripple current requirement.

where VIN is the input voltage, VOUT is the output voltage, fSW is the switching frequency, IOUTMAX is the maximum output

current, and KRIPPLE is the ripple factor. Typically, choose KRIPPLE = 30% to correspond to the peak-to-peak ripple

current being 30% of the maximum output current. With this inductor value (Table 1), the peak inductor current is IOUT

• (1 + KRIPPLE / 2). Make sure that this peak inductor current is less that the 2.4A current limit. Finally, select the

inductor core size so that it does not saturate at 2.4A.

VOUT

1.5V

1.8V

2.5V

3.3V

6.8µ H 6.8µ H 6.8µ H 8.5µ H

15µ H

Table 1: Typical Inductor Values

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