OUTPUT INDUCTORS & CAPACITORS FOR SW1 AND

SW2

There are several design considerations related to the selec-

tion of output inductors and capacitors:

•

Load transient response;

•

Stability;

•

Efficiency;

•

Output ripple voltage; and

•

Over-current ruggedness.

The LM26480 has been optimized for use with nominal values

2.2 µH and 10 µF. If other values are needed for the design,

please contact National Semiconductor sales with any con-

cerns.

INDUCTOR SELECTION FOR SW1 AND SW2

A nominal inductor value of 2.2 µH is recommended. It is im-

portant to guarantee the inductor core does not saturate

during any foreseeable operational situation.

Care should be taken when reviewing the different saturation

current ratings that are specified by different manufacturers.

Saturation current ratings are typically specified at 25ºC, so

ratings at maximum ambient temperature of the application

should be requested from the manufacturer.

There are two methods to choose the inductor saturation cur-

rent rating:

Recommended method:

The best way to guarantee the inductor does not saturate is

to choose an inductor that has saturation current rating

greater than the maximum LM26480 current limit of 2.4A. In

this case the device will prevent inductor saturation.

Alternate method:

If the recommended approach cannot be used, care must be

taken to guarantee that the saturation current is greater than

the peak inductor current:

30040471

I

Inductor saturation current at operating tempera-

ture

I

Peak inductor current during worst case conditions

I

Maximum average inductor current

I

Peak-to-Peak inductor current

V

Output voltage

V

Input voltage

L:

Inductor value in Henries at I

OUTMAX

F:

Switching frequency, Hertz

D:

Estimated duty factor

EFF:

Estimated power supply efficiency

I

transients, startup, high temperature, worst case conditions,

etc.

SUGGESTED INDUCTORS AND THEIR SUPPLIERS

Model

Vendor

Dimensions (mm)

DCR (max)

I

SATURATION

DO3314-222MX

Coilcraft

3.3 x 3.3 x 1.4

200 m

Ω

LPO3310-222MX

Coilcraft

3.3 x 3.3 x 1

150 m

Ω

ELL6PG2R2N

Panasonic

6.0 x 6.0 x 2.0

37 m

Ω

ELC6GN2R2N

Panasonic

6.0 x 6.0 x 1.5

53 m

Ω

CDRH2D14NP-2R2NC

Sumida

3.2 x 3.2 x 1.5

94 m

Ω

Note: Inductor Current Saturation values are estimates; inductor manufacturer should be contacted for guaranteed values.

OUTPUT CAPACITOR SELECTION FOR SW1 AND SW2

A ceramic output capacitor of 10 µF, 6.3V is recommended

with an ESR of about 2m

Ω or less.

Output ripple can be estimated from the vector sum of the

reactive (Capacitor) voltage component and the real (ESR)

voltage component of the output capacitor.

V

Estimated reactive output ripple

V

Estimated real output ripple

V

Estimated peak-to-peak output ripple

The output capacitor needs to be mounted as close as pos-

sible to the output pin of the device. For better temperature

performance, X7R or X5R types are recommended. DC bias

characteristics of ceramic capacitors must be considered

when selecting case sizes like 0805 and 0603.

DC bias characteristics vary from manufacturer to manufac-

turer and by case size. DC bias curves should be requested

from them as part of the capacitor selection process. ESR is

typically higher for smaller packages.

The output filter capacitor smooths out current flow from the

inductor to the load, helps maintain a steady output voltage

during transient load changes and reduces output voltage

ripple. These capacitors must be selected with sufficient ca-

pacitance and sufficiently low ESR to perform these functions.

Note that the output voltage ripple is dependent on the induc-

tor current ripple and the equivalent series resistance of the

output capacitor (ESR

dent as well as temperature dependent. The R

calculated with the applicable switching frequency and ambi-

ent temperature.

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