EC3293 Datasheet, PDF (8/11 Pages) E-CMOS Corporation – 3A, 18V, Synchronous Step-down DC/DC Converter

3A, 18V, Synchronous Step-down DC/DC Converter ^EC3293
Application Information(Cont.)
ripple current that will result in lower output ripple
voltage. However, the larger value inductor will have a
larger physical size, higher series resistance, and/or
lower saturation current. A good rule for determining
the inductance to use is to allow the peak-to-peak ripple
current in the inductor to be approximately 30% of the
maximum switch current limit. Also, make sure that the
peak inductor current is below the maximum switch
current limit. The inductance value can be calculated by:
L = [ V_OUT / (f_S × ΔI_L) ] × (1 − V_OUT/V_IN)
Where V_OUT is the output voltage, V_IN is the input voltage,
f_S is the switching frequency, and ΔI_L is the peak-to-peak
inductor ripple current.
Choose an inductor that will not saturate under the
maximum inductor peak current. The peak inductor
current can be calculated by:
I_LP = I_LOAD + [ V_OUT / (2 × f_S × L) ] × (1 − V_OUT/V_IN)
Where I_LOAD is the load current.
The choice of which style inductor to use mainly
depends on the price vs. size requirements and any EMI
requirements.
Optional Schottky Diode
switching current it requires an adequate ripple current
rating. The RMS current in the input capacitor can be
estimated by:
I_C1 = I_LOAD × [ (V_OUT/V_IN) × (1 − V_OUT/V_IN) ] ^1/2
The worst-case condition occurs at VI_N = 2V_OUT, where I_C1
= I_LOAD/2. For simplification, choose the input capacitor
whose RMS current rating greater than half of the
maximum load current.
The input capacitor can be electrolytic, tantalum or
ceramic. When using electrolytic or tantalum capacitors,
a small, high quality ceramic capacitor, i.e. 0.1μF, should
be placed as close to the IC as possible. When using
ceramic capacitors, make sure that they have enough
capacitance to provide sufficient charge to prevent
excessive voltage ripple at input. The input voltage
ripple for low ESR capacitors can be estimated by:
ΔV_IN = [ I_LOAD/(C1 × f_S) ] × (V_OUT/V_IN) × (1 − V_OUT/V_IN)
Where C1 is the input capacitance value.
Output Capacitor
During the transition between high-side switch and
low-side switch, the body diode of the low-side power
MOSFET conducts the inductor current. The forward
voltage of this body diode is high. An optional Schottky
diode may be paralleled between the SW pin and GND
pin to improve overall efficiency. Table 1 lists example
Schottky diodes and their Manufacturers.
Part
Number
B130
SK13
MBRS130
Voltage and
Current Rating
30V, 1A
30V, 1A
30V, 1A
Vendor
Diodes Inc.
Diodes Inc.
International
Rectifier
Table 1. Diode Selection guide
Input Capacitor
The input current to the step-down converte is
discontinuous, therefore a capacitor is required
to supply the AC current to the step-down converter
while maintaining the DC input voltage. Use low ESR
capacitors for the best performance. Ceramic capacitors
are preferred,but tantalum or low-ESR electrolytic
capacitors may also suffice. Choose X5R or X7R
dielectrics when using ceramic capacitors.
Since the input capacitor (C1) absorbs the input
The output capacitor is required to maintain the DC
output voltage. Ceramic, tantalum, or low ESR
electrolytic capacitors are recommended. Low ESR
capacitors are preferred to keep the output voltage
ripple low. The output voltage ripple can be estimated
by:
ΔV_OUT = [ V_OUT/(f_S × L) ] × (1 − V_OUT/V_IN)
× [ R_ESR + 1 / (8 × f_S × C2) ]
Where C2 is the output capacitance value and R_ESR is the
equivalent series resistance (ESR) value of the output
capacitor.
In the case of ceramic capacitors, the impedance at the
switching frequency is dominated by the capacitance.
The output voltage ripple is mainly caused by the
capacitance. For simplification, the output voltage ripple
can be estimated by:
ΔV_OUT = [ V_OUT/(8 × f_S2 × L × C2) ] × (1 − V_OUT/V_IN)
In the case of tantalum or electrolytic capacitors, the
ESR dominates the impedance at the switching
frequency. For simplification, the output ripple can be
approximated to:
ΔV_OUT = [ V_OUT/(f_S × L) ] × (1 − V_OUT/V_IN) × R_ESR
The characteristics of the output capacitor also affect
the stability of the regulation system. The EC3293
E-CMOS Corp. (www.ecmos.com.tw)
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3L03N-Rev.P001