WIDE INPUT 3A STEP-DOWN CONVERTER
FSP3128
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
R
FB1
and R
FB2
based on the output voltage. Typically, use RFB2≈10KΩ and determine R
FB1
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 V
IN
is the input voltage, V
OUT
is the output voltage, f
SW
is the switching frequency, I
OUTMAX
is the maximum output
current, and K
RIPPLE
is the ripple factor. Typically, choose K
RIPPLE
= between 20% and 30% to correspond to the
peak-to-peak ripple current being a percentage of the maximum output current. With this inductor value (Table 1), the
peak inductor current is I
OUT
• (1 + K
RIPPLE
/ 2). Make sure that this peak inductor current is less that the 5A current limit.
Finally, select the inductor core size so that it does not saturate at 5A.
V
O U T
L
1.5V
6.8µ H
1.8V
2.5V
3.3V
5V
15µ H
6.8µ H 6.8µ H 8.5µ H
Table 1: Typical Inductor Values
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2007-1-4
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