FSP3126 Datasheet, PDF (4/9 Pages) FOSLINK SEMICONDUCTOR CO.,LTD – WIDE INPUT 2A STEP DOWN CONVERTER

„ APPLICATION INFORMATION
Output Voltage Setting
WIDE INPUT 2A STEP DOWN CONVERTER
FSP3126
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:
R_FB1
=
R
FB2
⎜⎜⎝⎛
V_OUT
1.293V
− 1_{⎟⎟⎠⎞}
(1)
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:
L = ^{VOUT • (VIN − VOUT )}
V_IN f_SW I_OUTMAX K _{RIPPLE (2)}
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 = 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 I_OUT • (1 + K_RIPPLE / 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.
V_OUT 1.5V 1.8V 2.5V 3.3V 5V
L 6.8µH 6.8µH 10µH 15µH 22µH
Table 1. Typical Inductor Values
Input Capacitor
The input capacitor needs to be carefully selected to maintain sufficiently low ripple at the supply input of the
converter. A low ESR capacitor is highly recommended. Since large current flows in and out of this capacitor during
switching, its ESR also affects efficiency.
The input capacitance needs to be higher than 10µF. The best choice is the ceramic type; however, low ESR tantalum
or electrolytic types may also be used provided that the RMS ripple current rating is higher than 50% of the output
current. The input capacitor should be placed close to the IN and G pins of the IC, with the shortest traces possible. In
the case of tantalum or electrolytic types, they can be further away if a small parallel 0.1µF ceramic capacitor is
placed right next to the IC.
Output Capacitor
The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is:
+
V_IN
V_RIPPLE = I_OUTMAX K _RIPPLE R_ESR
28 • f _SW ² LC_OUT
(3)
where I_OUTMAX is the maximum output current, K_RIPPLE is the ripple factor, R_ESR is the ESR of the output capacitor, f_SW is
the switching frequency, L is the inductor value, and C_OUT is the output capacitance. In the case of ceramic output
capacitors, R_ESR is very small and does not contribute to the ripple. Therefore, a lower capacitance value can be used
for ceramic capacitors. In the case of tantalum or electrolytic capacitors, the ripple is dominated by R_ESR multiplied by
the ripple current. In that case, the output capacitor is chosen to have sufficiently low ESR.
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2007-5-28