LM2597 datasheet(13/34 Pages) NSC

13 / 34 page

LM2597/LM2597HV 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 220 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 220 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 2. 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 volt-

age 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 2, 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

section, select a capacitor from the list of through hole elec-

trolytic or surface mount tantalum types from four different

capacitor manufacturers. It is recommended that both the

manufacturers 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.

82 µF

50V

Panasonic HFQ Series

120 µF

50V

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 50V rating was chosen because it has a lower

ESR which provides a lower output ripple voltage.

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 infor-

mation.

4. Feedforward Capacitor (C

FF) (See Figure 12)

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

tional capacitor is required. The compensation capacitor is

typically between 50 pF and 10 nF, and is wired in parallel with

the output voltage setting resistor, R

2. It provides additional

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

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 2 contains feed forward capacitor

values for various output voltages. In this example,a1nF

capacitor is needed.

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