LSP5502

2A Synchronous Step Down DC/DC Converter

4/11

Rev. 1.7

www.liteon-semi.com

APPLICATION INFORMATION

Output Voltage Setting

Figure1. 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 following

equation:

(1)

Table 1-Recommended Resistance Values

VOUT

RFB1

RFB2

1.0V

1.0kΩ

12kΩ

1.2V

3.0kΩ

10kΩ

1.8V

9.53kΩ

10kΩ

2.5V

16.9kΩ

10kΩ

3.3V

26.1kΩ

10kΩ

5V

44.2kΩ

10kΩ

12V

121kΩ

10kΩ

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 the ripple current requirement:

RIPPLE

OUTMAX

SW

IN

OUT

IN

OUT

K

I

f

V

)

V

V

(

V

L

−

•

=

(2)

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, the peak inductor current is IOUT • (1 + KRIPPLE / 2). Make sure that this peak inductor current

is less that the 3A current limit. Finally, select the inductor core size so that it does not saturate at 3A. Typical

inductor values for various output voltages are shown in Table 1.

VOUT 1.0V 1.2V 1.5V 1.8V 2.5V 3.3V

5V

L

4.7uH 4.7uH 6.8µH6.8µH 10µH 10µH 15µH

Table 1. Typical Inductor Values

Input Capacitor

Free Datasheet http://www.nDatasheet.com