Input Capacitor Selection

Boost Converter Design Procedure

in

out

in

out

swpeak

out

in

swpeak

V

1. Converter Duty Cycle:

D = 1

V

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

-

´

æ

ö

-

´ -

ç

÷

´

è

ø

´

´

-

(9)

Inductor Selection (Boost Converter)

TPS65166

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

µF input

capacitors as close as possible to these pins. Another 10

µF 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

µF

capacitors close to the device, it is recommended to add an additional 1

µF or 4.7µF 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

µF 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

µF/16V

Murata, GRM31CR71C106KAC7

10

µF/16V

Taiyo Yuden, EMK325BJ106MN

10

µF/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,

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

µH to 15µH inductors, a 10µH 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

µH

Sumida CDRH103R

10.3 × 10.5 × 3.1

3.2 A/45 m

Ω

10

µH

Sumida CDRH8D38

8.3 × 8.3 × 4.0

3 A/38 m

Ω

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