CHOOSING AN INDUCTANCE VALUE

The current ripple present in the output filter inductor is

determined by the input voltage, output voltage, switching

frequency and inductance according to the following equa-

tion:

where

∆I

cycle V

stage, V

switching frequency and L

filter inductor. Knowing the current ripple is important for

inductor selection since the peak current through the induc-

tor is the load current plus one half the ripple current. Care

must be taken to ensure the peak inductor current does not

reach a level high enough to trip the current limit circuitry of

the LM2853. As an example, consider a 5V to 1.2V conver-

sion and a 550 kHz switching frequency. According to Table

1, a 4.7 µH inductor may be used. Calculating the expected

peak-to-peak ripple,

The maximum inductor current for a 3A load would therefore

be 3A plus 177 mA, 3.177A. As shown in the ripple equation,

the current ripple is inversely proportional to inductance.

OUTPUT FILTER INDUCTORS

Once the inductance value is chosen, the key parameter for

selecting the output filter inductor is its saturation current

(I

turer as the current at which the inductance of the coil falls to

a certain percentage of the nominal inductance. The I

an inductor used in an application should be greater than the

maximum expected inductor current to avoid saturation. Be-

low is a table of inductors that are suitable in LM2853

applications.

TABLE 2. Recommended Inductors

Inductance

Part Number

Vendor

4.7 µF

DO3308P-472ML

Coilcraft

4.7 µF

DO3316P-472ML

Coilcraft

4.7 µF

MSS1260-472ML

Coilcraft

5.2 µF

MSS1038-522NL

Coilcraft

5.6 µF

MSS1260-562ML

Coilcraft

6.8 µF

DO3316P-682ML

Coilcraft

6.8 µF

MSS1260-682ML

Coilcraft

OUTPUT FILTER CAPACITORS

The recommended capacitors that may be used in the output

filter with the LM2853 are limited in value and ESR range

according to Table 1.

Below are some examples of capacitors that can typically be

used in an LM2853 application.

TABLE 3. Recommended Capacitors

Capacitance (µF)

Part Number

Chemistry

Vendor

100

594D107X_010C2T

Tantalum

Vishay-Sprague

100

593D107X_010D2_E3

Tantalum

Vishay-Sprague

100

TPSC107M006#0075

Tantalum

AVX

100

NOSD107M006#0080

Niobium Oxide

AVX

100

NOSC107M004#0070

Niobium Oxide

AVX

120

594D127X_6R3C2T

Tantalum

Vishay-Sprague

150

594D157X_010C2T

Tantalum

Vishay-Sprague

150

595D157X_010D2T

Tantalum

Vishay-Sprague

150

591D157X_6R3C2_20H

Tantalum

Vishay-Sprague

150

TPSD157M006#0050

Tantalum

AVX

150

TPSC157M004#0070

Tantalum

AVX

150

NOSD157M006#0070

Niobium Oxide

AVX

220

594D227X_6R3D2T

Tantalum

Vishay-Sprague

220

591D227X_6R3D2_20H

Tantalum

Vishay-Sprague

220

591D227X_010D2_20H

Tantalum

Vishay-Sprague

220

593D227X_6R3D2_E3

Tantalum

Vishay-Sprague

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