ACT4524
Rev 1.1, 22-Feb-16
APPLICATIONS INFORMATION
Inductor Selection
Input Capacitor
The inductor maintains a continuous current to the
output load. This inductor current has a ripple which
is determined by the inductance value.
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.
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:
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 VIN and GND pins of the IC, with the shortest
traces as possible. In the case of tantalum or
electrolytic types, they can be placed a little bit
away of IC if a paralleled ceramic capacitor is
placed right next to the IC.
_
(
)
VOUT × V VOUT
IN
(1)
L =
VINfSW ILOADMAXKRIPPLE
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 = 30% to correspond to the peak-to-
peak ripple current being 30% of the maximum load
current.
Output Capacitor
The output capacitor also needs to 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:
_
(
)
VOUT × V VOUT
IN
ILPK _
=
VIN
PK
(5)
L×V ×fSW
VRIPPLE IOUTMAX KRIPPLE RESR
IN
28 fSW 2LCOUT
(2)
(3)
The peak inductor current is estimated as:
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 inductance, and COUT is the output capacitance.
In the case of ceramic output capacitors, RESR is very
small and only contributes a very small portion of the
ripple. Therefore, a lower 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 should be chosen to have
sufficiently low ESR.
1
ILPK = ILOADMAX
+
ILPK _
PK
2
The selected inductor should not saturate at ILPK.
The maximum output current is calculated as:
1
_
IOUTMAX = ILIM
ILPK _
PK
(4)
2
LLIM is the internal current limit.
External High Voltage Bias Diode
For ceramic type output capacitor, typically choose
a capacitance of about 22µF. For tantalum or
electrolytic capacitors, choose a capacitor with less
than 50mΩ ESR. A 330µF or 470µF electrolytic
capacitor is recommended.
It is recommended that an external High Voltage
Bias diode be added when the system has a 5V
fixed input or the power supply generates a 5V
output. This helps improve the efficiency of the
regulator. The High Voltage Bias diode can be a
low cost one such as IN4148 or BAT54.
Rectifier Diode
Use a low forward voltage drop (Vf<0.5V) 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.
5V
HSB
22nF
ACT4524
SW
Innovative PowerTM
- 7 -
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