ACT4527
Rev 1, December 28th, 2015
APPLICATIONS INFORMATION
current. The input capacitor should be placed close
to the IN and GND pins of the IC, with the shortest
traces possible. In the case of tantalum or
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.
electrolytic types,
a
ceramic capacitor is
recommended to parallel with tantalum or
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
VIN
VRIPPLE IOUTMAX K RIPPLE RESR
(5)
28 fSW 2 LCOUT
electrolytic capacitor, which should be placed right
next to the IC.
based on ripple current
requirement:
_
V
OUT × V VOUT
IN
L =
Output Capacitor
(1)
VINfSWILOADMAXKRIPPLE
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
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
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. In the case of ceramic output
capacitors, RESR is very small and does not
factor. Typically, choose KRIPPLE
=
30% to
correspond to the peak-to-peak ripple current being
30% of the maximum load current.
_
With
a
selected
VOUT × VIN VOUT
contribute to the ripple. Therefore,
a
lower
ILPK _
=
inductor value the
p e a k - t o - p e a k
(2)
(3)
PK
L×VIN ×fSW
capacitance value can be used for ceramic type. In
the case of tantalum or electrolytic capacitors, the
ripple is dominated by RESR multiplied by the ripple
current. In that case, the output capacitor is chosen
to have sufficiently low ESR.
inductor current is estimated as:
1
ILPK = ILOADMAX
+
ILPK_
PK
2
The peak inductor current is estimated as:
For ceramic output capacitor, typically choose a
capacitance of about 22µF. For tantalum or
electrolytic capacitors, choose a capacitor with less
than 50mÙ ESR. An 330µF or 470µF electrolytic
capacitor is recommended.
1
_
IOUTMAX = ILIM
ILPK _
PK
(4)
2
The selected inductor should not saturate at
ILPK.
The maximum output current is calculated as:
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.
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 current flows in and out
of this capacitor during switching, its ESR also
affects efficiency.
PCB Load
Trace
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
Kevin Sense
Traces
Sense
Resistor
Innovative PowerTM
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