ACT4529
Rev 4, 06-Jan-2017
APPLICATIONS INFORMATION
recommended to parallel with tantalum or
electrolytic capacitor, which should be placed right
next 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.
Output Capacitor
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 output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
(VIN VOUT )VOUT
8 fSW 2 LCOUT VIN
(5)
VRIPPLE IOUTMAXKRIPPLERESR
_
V
OUT × V VOUT
IN
(1)
L =
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.
VINfSWILOADMAXKRIPPLE
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.
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
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.
With a selected inductor value the peak-to-peak
inductor current is estimated as:
_
VOUT × VIN VOUT
(2)
(3)
ILPK _
=
PK
L×VIN ×fSW
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. If an 330uF or 470uF electrolytic
cap or tantalum cap is used, where ripple is
dominantly caused by ESR, an 2.2uF ceramic in
parallel is recommended.
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
Rectifier Schottky Diode
LLIM is the internal current limit.
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.
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 GND pins of the IC, with the shortest
traces possible. In the case of tantalum or
electrolytic types,
a
ceramic capacitor is
Innovative PowerTM
- 8 -
www.active-semi.com
Copyright © 2015-2017 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
