
Liteon Semiconductor Corporation
LSP3129
Wide Input 2A Step Down Converter
Rev1.0
4/10
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≈10K 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
5V
L
6.8 H 6.8 H 6.8 H 8.5 H
15 H
Table 1: Typical Inductor Values