ΔI

L

I

= I

+

L

OUT

2

V

V

OUT

OUT

x 1 -

η

V

x η

IN

I = I

+

L

OUT

2 x

x L

f

æ

ö

ç

÷

ç

÷

è

ø

TLV62090

www.ti.com

SLVSBB9B – MARCH 2012 – REVISED APRIL 2012

APPLICATION INFORMATION

DESIGN PROCEDURE

The first step is the selection of the output filter components. To simplify this process, and Table 2 outline

possible inductor and capacitor value combinations.

Table 2. Output Filter Selection

10

22

47

100

150

0.47

√

√

√

√

1.0

√

√

√

√

2.2

√

√

√

√

√

3.3

(1)

Inductor tolerance and current de-rating is anticipated. The effective inductance can vary by +20% and

–30%.

(2)

Capacitance tolerance and bias voltage de-rating is anticipated. The effective capacitance can vary by

+20% and –50%.

(3)

Typical application configuration. Other check mark indicates alternative filter combinations

Inductor Selection

The inductor selection is affected by several parameter like inductor ripple current, output voltage ripple,

transition point into Power Save Mode, and efficiency. See Table 3 for typical inductors.

Table 3. Inductor Selection

INDUCTOR VALUE

COMPONENT SUPPLIER

SIZE (LxWxH mm)

Isat/DCR

0.6 µH

Coilcraft XAL4012-601

4 x 4 x 2.1

7.1A/9.5 m

Ω

1 µH

Coilcraft XAL4020-102

4 x 4 x 2.1

5.9A/13.2 m

Ω

1 µH

Coilcraft XFL4020-102

4 x 4 x 2.1

5.1 A/10.8 m

Ω

0.47 µH

TOKO DFE252012 R47

2.5 x 2 x 1.2

3.7A/39 m

Ω

1 µH

TOKO DFE252012 1R0

2.5 x 2 x 1.2

3.0A/59 m

Ω

0.68 µH

TOKO DFE322512 R68

3.2 x 2.5 x 1.2

3.5A/37 m

Ω

1 µH

TOKO DFE322512 1R0

3.2 x 2.5 x 1.2

3.1A/45 m

Ω

In addition, the inductor has to be rated for the appropriate saturation current and DC resistance (DCR). The

inductor needs to be rated for a saturation current as high as the typical switch current limit, of 4.6 A or according

to Equation 7 and Equation 8. Equation 7 and Equation 8 calculate the maximum inductor current under static

load conditions. The formula takes the converter efficiency into account. The converter efficiency can be taken

from the data sheet graph`s or 80% can be used as a conservative approach. The calculation must be done for

the maximum input voltage where the peak switch current is highest.

(7)

(8)

where

ƒ = Converter switching frequency (typical 1.4 MHz)

L = Selected inductor value

η = Estimated converter efficiency (use the number from the efficiency curves or 0.80 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% needs to be added to cover for load transients during operation.

Copyright © 2012, Texas Instruments Incorporated

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