LM2641 datasheet(16/18 Pages) NSC

16 / 18 page

Design Procedure (Continued)

It is also recommended that a 0.1µF ceramic capacitor be

placed from V

IN to ground for high frequency bypassing, lo-

cated as close as possible to the V

IN pin.

OUTPUT CAPACITORS

The output capacitor(s) are critical in loop stability (covered

in a previous section) and also output voltage ripple.

The types best suited for use as output capacitors are alumi-

num electrolytics and solid Tantalum.

Aluminum Electrolytics

The primary advantage of aluminum electrolytics is that they

typically give the maximum capacitance-to-size ratio, and

they are reasonably priced. However, it must be noted that

aluminum electrolytics used in high-performance switching

regulator designs must be high frequency, low ESR types

such as Sanyo OSCON or Panasonic HFQ which are spe-

cifically designed for switching applications. Capacitors such

as these with good high frequency (

≥ 100kHz) specifications

are not cheap.

Aluminum electrolytic capacitors should generally not be

used in switching regulator applications where the ambient

temperature goes below 0˚C. A typical low-voltage aluminum

electrolytic has an ESR vs. Temperature curve that is fairly

flat from 25˚C to 125˚C. However, a temperature change

from 25˚C to 0˚C will approximately double the ESR, and it

will double again going from 0˚C down to −20˚C.

Tantalum

Solid Tantalum capacitors are best in applications which

must operate over a wide temperature range. A good quality

Tantalum will typically exhibit less than 2:1 change in ESR

over the temperature range of +125˚C to −40˚C. Recom-

mended types are Sprague 593D, Sprague 594D, and AVX

TPS series.

Selecting An Output Capacitor

The required value of output capacitance is directly related

to the specification for the maximum amount of output volt-

age ripple allowed in the application. Since ESR effects the

ripple voltage, it is important to have a guideline for ESR.

The maximum allowed ESR can be calculated as follows.

RIPPLE = IRIPPLE *ESR(max)

Using V = Ldi/dt

OUT = L *IRIPPLE/{(1−D)TS} = L *IRIPPLE *FS/(1−D)

RIPPLE = VOUT*(1−D)/)L *FS)

ESR(max) = V

RIPPLE/IRIPPLE

A reasonable value for C

OUT can be obtained by choosing

capacitors with net ESR less than 1⁄2 of ESR(max).

Hence,

ESR(max) = V

RIPPLE*L* FS/{VOUT(1−D)}

The value of C

OUT necessary to meet the voltage ripple

specification can be found using the approximation:

Where:

RIPPLE is the inductor ripple current.

RIPPLE is the output ripple voltage.

ESR is the equivalent series resistance of the output capaci-

tor.

F is the switching frequency, F

S = 1/FS.

The ESR term predominates in determining output ripple

voltage. Good quality Tantalum capacitors have guaranteed

maximum specifications for ESR, but the typical values for

ESR are usually considerably lower than the maximum limit.

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