LM2596 datasheet(14/31 Pages) NSC

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LM2596 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

3. Output Capacitor Selection (C

OUT)

A. In the majority of applications, low ESR electrolytic or solid

tantalum capacitors between 82 µF and 820 µF provide the

best results. This capacitor should be located close to the IC

using short capacitor leads and short copper traces. Do not

use capacitors larger than 820 µF. For additional informa-

tion, see section on output capacitors in application in-

formation section.

B. To simplify the capacitor selection procedure, refer to the

quick design table shown in

Figure 3. This table contains

different output voltages, and lists various output capacitors

that will provide the best design solutions.

C. The capacitor voltage rating should be at least 1.5 times

greater than the output voltage, and often much higher voltage

ratings are needed to satisfy the low ESR requirements

needed for low output ripple voltage.

3. Output Capacitor SeIection (C

OUT)

A. See section on C

OUT in Application Information section.

B. From the quick design table shown in

Figure 3, locate the

output voltage column. From that column, locate the output

voltage closest to the output voltage in your application. In this

example, select the 24V line. Under the output capacitor sec-

tion, select a capacitor from the list of through hole electrolytic

or surface mount tantalum types from four different capacitor

manufacturers. It is recommended that both the manufactur-

ers and the manufacturers series that are listed in the table be

used.

In this example, through hole aluminum electrolytic capacitors

from several different manufacturers are available.

220 µF/35V Panasonic HFQ Series

150 µF/35V Nichicon PL Series

C. For a 20V output, a capacitor rating of at least 30V or more

is needed. In this example, either a 35V or 50V capacitor

would work. A 35V rating was chosen, although a 50V rating

could also be used if a lower output ripple voltage is needed.

Other manufacturers or other types of capacitors may also be

used, provided the capacitor specifications (especially the 100

kHz ESR) closely match the types listed in the table. Refer to

the capacitor manufacturers data sheet for this information.

4. Feedforward Capacitor (C

FF) (See Figure 1)

For output voltages greater than approximately 10V, an addi-

tional capacitor is required. The compensation capacitor is

typically between 100 pF and 33 nF, and is wired in parallel

with the output voltage setting resistor, R

2. It provides addi-

tional stability for high output voltages, low input-output volt-

ages, and/or very low ESR output capacitors, such as solid

tantalum capacitors.

This capacitor type can be ceramic, plastic, silver mica, etc.

(Because of the unstable characteristics of ceramic capacitors

made with Z5U material, they are not recommended.)

4. Feedforward Capacitor (C

FF)

The table shown in

Figure 3 contains feed forward capacitor

values for various output voltages. In this example, a 560 pF

capacitor is needed.

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