ACT4530
Rev 1.0, 19-Nov-2018
APPLICATIONS INFORMATION
capacitor in parallel with a tantalum or electrolytic.
This combination provides the EMI and noise
performance. The input capacitor must be placed
close to the IN and GND pins of the IC, with the
shortest traces possible. If using a tantalum or
electrolytic capacitor in parallel with ceramic
capacitor, the ceramic capacitor must be placed
closer to the IC.
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:
Output Capacitor
The ACT4530 output capacitance must be split
between the left and right side of the output current
sense resistor. The left side of the current sense
resistor (CSP pin) requires a 22uF ceramic
capacitor. The right side of the current sense
resistor should contain enough capacitance to keep
the output voltage ripple below the require level.
(1)
Where VIN is the input voltage, VOUT is the output
voltage, fSW is the switching frequency, ILOADMAX is
the maximum load current, and KRIPPLE is the ripple
factor. Typically, choose KRIPPLE
correspond to the peak-to-peak ripple current being
30% of the maximum load current.
=
30% to
(5)
This output capacitance should have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
With a selected inductor value the peak-to-peak
inductor current is estimated as:
(2)
Where IOUTMAX is the maximum output current,
KRIPPLE is the ripple factor, RESR is the ESR of the
output capacitor, fSW is the switching frequency, L is
the inductor value, and COUT is the output
capacitance. From the equation above, VRIPPLE is
the combination of ESR and real capacitance.
The peak inductor current is estimated as:
(3)
(4)
In the case of ceramic output capacitors, RESR is very
small and does not contribute to the ripple.
Therefore, a lower capacitance value can be used. In
the case of tantalum or electrolytic capacitors, the
ripple is dominated by RESR. In this case, the output
capacitor must chosen to have sufficiently low ESR.
The selected inductor should not saturate at ILPK.
The maximum output current is calculated as:
For ceramic output capacitors, typically choose a
capacitance of about 22µF. For tantalum or
electrolytic capacitors, choose a capacitor with less
than 50mΩ ESR. If an 330uF or 470uF electrolytic
cap or tantalum cap is used and the output voltage
ripple is dominated by ESR, add a 2.2uF ceramic in
parallel with the tantalum or electrolytic.
LLIM is the internal current limit.
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 currents flow in and out
of this capacitor during switching, its ESR also
affects efficiency.
Rectifier Schottky Diode
Use a Schottky diode as the rectifier to conduct
current when the High-Side Power Switch is off.
The Schottky diode must have current rating higher
than the maximum output current and a reverse
voltage rating higher than the maximum input
voltage. Further more, the low forward voltage
Schottky is preferable for high efficiency and
smoothly operation.
The input capacitance needs to be higher than
10µF. The best choice is a ceramic capacitor.
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. Active Semi recommends using a ceramic
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