2

L

PEAK

OUT

I

I

I

'

(6)

where:

• L = inductance

• η = estimated efficiency (use the value from the efficiency curves or 0.9 as an conservative assumption)

Note

The calculation must be done for the maximum input voltage of the application.

Calculating the maximum inductor current using the actual operating conditions gives the minimum saturation

current. A margin of 20% is recommended to be added to cover for load transients during operation.

See Table 8-4 for typical inductors.

Table 8-4. Inductor Selection

INDUCTOR VALUE

MANUFACTURER

PART NUMBER

SIZE (L X W X H IN mm)

ISAT/DCR (30% DROP)

2.2 µH

Coilcraft

XGL4020-222

4 x 4 x 2.1

6.2 A / 19.5 mΩ

2.2 µH

Coilcraft

XGL4030-222

4 x 4 x 3.1

7 A / 13.5 mΩ

2.2 µH

Wurth Elektronik

74438356022

4.1 x 4.1 x 2.1

5.2 A / 35 mΩ

2.2 µH

Wurth Elektronik

74438357022

4.1 x 4.1 x 3.1

7 A / 26 mΩ

2.2 µH

MuRata

DFE322520FD-2R2M=P2 3.2 x 2.5 x 2

5 A / 46 mΩ

4.7 µH

Coilcraft

XGL4020-472

4 x 4 x 2.1

4.1 A / 43.0 mΩ

4.7 µH

Coilcraft

XGL4030-472

4 x 4 x 3.1

4.4 A / 28.5 mΩ

4.7 µH for TPS62912 only MuRata

DFE322520FD-4R7M=P2 3.2 x 2.5 x 2

3.4 A / 98 mΩ

8.2.2.2.3 Output Capacitor Selection

The effective output capacitance can range from 40 μF (minimum) up to 200 μF (maximum) for a single L-C

system design. When using a second L-C filter, the first L-C filter must have output capacitance between 40 μF

and 80 μF, the second stage L-C filter (if used) must have at least 20 μF of capacitance, and the total

capacitance for both L-C filters must be less than 200 μF. Load transient testing and measuring the bode plot are

good ways to verify stability.

Ceramic capacitors (X5R or X7R) are recommended. Ceramic capacitors have a DC-Bias effect, which has a

strong influence on the final effective capacitance. Choose the right capacitor carefully in combination with

considering its package size and voltage rating. The ESR and ESL of the output capacitor are also important

considerations in selecting the output capacitors for low noise applications. Smaller package sizes typically have

lower ESL and ESR. 0805 or smaller packages are recommended, as long as they provide the required

capacitance and voltage rating for stable operation. Table 8-5 lists recommended output capacitors.

Table 8-5. Recommended Output Capacitors

CAPACITOR TYPE

CAPACITOR VALUE

MANUFACTURER

VOLTAGE (V)

PACKAGE

Bulk Capacitor

22 μF, X7S

TDK C2012X7S1A226M125AC

10

0805

Bulk Capacitor

47 μF, X7R

Murata GRM32ER71A476ME15L

10

1210

8.2.2.2.4 Ferrite Bead Selection for Second L-C Filter

Using a ferrite bead for the second stage L-C filter minimizes the external component count because most of the

noise sensitive circuits use a RF bead for high frequency attenuation as a default component at their inputs.

It is important to select a ferrite bead with sufficiently high inductance at full load, and with low DC resistance

(below 10 mΩ) to keep the converter efficiency as high as possible. The ferrite bead inductance decreases with

increased load current. Therefore, the ferrite bead should have a current rating much higher than the desired

load current.

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