
Input Capacitor Selection
Boost Converter Design Procedure
in
out
in
out
swpeak
out
in
swpeak
V
1. Converter Duty Cycle:
D = 1
V
D
2. Maximum output current: I
= I
(1 D)
2fs
L
I
V
D
3. Peak switch current:
I
=
+
2fs
L
1 D
h
-
-
-
÷
è
-
(9)
Inductor Selection (Boost Converter)
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For good input voltage filtering, low ESR ceramic capacitors are recommended. All input voltages (AVIN, VIN1,
2, 3) are shorted internally. It is recommended to short AVIN, VIN1, and VIN2 externally on the PCB by a thick
conducting path to avoid high currents between the VIN pins inside the device and to place two 10
F input
capacitors as close as possible to these pins. Another 10
F input capacitor should be placed close to VIN3. For
better input voltage filtering the input capacitor values can be increased. If it is not possible to place the 10
F
capacitors close to the device, it is recommended to add an additional 1
F or 4.7F capacitor which should be
placed next to the input pins. To reduce power losses at the external isolation switch, a filter capacitor C3 at the
input terminal of the inductor is required. To minimize possible audible noise problems, two 10
F capacitors in
parallel are recommended. More capacitance further reduces the ripple current across the isolation switch. See
Table 2 for input capacitor selection.
Table 2. Input Capacitor Selection
CAPACITOR
COMPONENT SUPPLIER
10
F/16V
Murata, GRM31CR71C106KAC7
10
F/16V
Taiyo Yuden, EMK325BJ106MN
10
F/16V
Murata, GRM31CR61C106KA88
The first step in the design procedure is to verify whether the maximum possible output current of the boost
converter supports the specific application requirements. To simplify the calculation, the fastest approach is to
estimate the converter efficiency by taking the efficiency numbers from the provided efficiency curves or to use a
worst case assumption for the expected efficiency, e.g., 90%. The calculation must be made with the minimum
assumed input voltage where peak switch current is the highest. The inductor and external Schottky diode has to
be able to handle this current.
With,
Iswpeak = Converter peak switch current (minimum switch current limit = 4.2 A)
fs = Converter switching frequency (typical 750 kHz)
L = Selected inductor value
η = Estimated converter efficiency (use the number from the efficiency curves or 0.9 as an assumption)
The boost converter is able to operate with 6.8
H to 15H inductors, a 10H inductor is typical. The main
parameter for inductor selection is the saturation current of the inductor, which should be higher than the peak
switch current as calculated in the Design Procedure section with additional margin to cover for heavy load
transients. The alternative more conservative approach is to choose an inductor with saturation current at least
as high as the minimum switch current limit of 4.2A. Another important parameter is the inductor dc resistance.
Usually the lower the dc resistance the higher the efficiency. For a boost converter where the inductor is the
energy storage element, the type and core material of the inductor influences the efficiency as well. The
efficiency difference among inductors can vary up to 10%. Possible inductors are shown in
Table 3.Table 3. Inductor Selection Boost Converter
INDUCTOR VALUE
COMPONENT SUPLIER
SIZE (L×W×H mm)
Isat/DCR
10
H
Sumida CDRH103R
10.3 × 10.5 × 3.1
3.2 A/45 m
10
H
Sumida CDRH8D38
8.3 × 8.3 × 4.0
3 A/38 m
Copyright 2009, Texas Instruments Incorporated
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