LM2641 datasheet(15/18 Pages) NSC

15 / 18 page

Design Procedure (Continued)

The data sheet for the inductor must be reviewed carefully to

verify that the selected component will have the desired in-

ductance at the frequency and current for the application.

Current Rating

This specification may be the most confusing of all when

picking an inductor, as manufacturers use different methods

for specifying an inductor’s current rating.

The current rating specified for an inductor is typically given

in RMS current, although in some cases a peak current rat-

ing will also be given (usually as a multiple of the RMS rat-

ing) which gives the user some indication of how well the in-

ductance operates in the saturation region.

Other things being equal, a higher peak current rating is pre-

ferred, as this allows the inductor to tolerate high values of

ripple current without significant loss of inductance.

In the some cases where the inductance vs. current curve is

relatively flat, the given current rating is the point where the

inductance drops 10% below the nominal value. If the induc-

tance varies a lot with current, the current rating listed by the

manufacturer may be the “center point” of the curve. This

means if that value of current is used in your application, the

amount of inductance will be less than the specified value.

DC Resistance

The DC resistance of the wire used in an inductor dissipates

power which reduces overall efficiency. Thicker wire de-

creases resistance, but increases size, weight, and cost. A

good tradeoff is achieved when the inductor’s copper wire

losses are about 2% of the maximum output power.

Selecting An Inductor

Determining the amount of inductance required for an appli-

cation can be done using the formula:

Where:

IN is the maximum input voltage.

OUT is the output voltage.

F is the switching frequency, F

OSC

RIPPLE is the inductor ripple current. In general, a good value

for this is about 30% of the DC output current.

It can be seen from the above equation, that increasing the

switching frequency reduces the amount of required induc-

tance proportionally. Of course, higher frequency operation

is typically less efficient because switching losses become

more predominant as a percentage of total power losses.

It should also be noted that reducing the inductance will in-

crease inductor ripple current (other terms held constant).

This is a good point to remember when selecting an inductor:

increased ripple current increases the FET conduction

losses, inductor core losses, and requires a larger output ca-

pacitor to maintain a given amount of output ripple voltage.

This means that a cheaper inductor (with less inductance at

the operating current of the application) will cost money in

other places.

INPUT CAPACITORS

The switching action of the high-side FET requires that high

peak currents be available to the switch or large voltage tran-

sients will appear on the V

IN line. To supply these peak cur-

rents, a low ESR capacitor must be connected between the

drain of the high-side FET and ground. The capacitor must

be located as close as possible to the FET (maximum dis-

tance = 0.5 cm).

A solid Tantalum or low ESR aluminum electrolytic can be

used for this capacitor. If a Tantalum is used, it must be able

to withstand the turn-ON surge current when the input power

is applied. To assure this, the capacitor must be surge tested

by the manufacturer and guaranteed to work in such applica-

tions.

Caution: If a typical off-the-shelf Tantalum is used that has

not been surge tested, it can be blown during power-up and

will then be a dead short. This can cause the capacitor to

catch fire if the input source continues to supply current.

Voltage Rating

For an aluminum electrolytic, the voltage rating must be at

least 25% higher than the maximum input voltage for the ap-

plication.

Tantalum capacitors require more derating, so it is recom-

mended that the selected capacitor be rated to work at a

voltage that is about twice the maximum input voltage.

Current Rating

Capacitors are specified with an RMS current rating. To de-

termine the requirement for an application, the following for-

mula can be used:

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