LM2598

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SNVS125C – MARCH 1998 – REVISED APRIL 2013

Table 1. LM2598 Series Buck Regulator Design Procedure (Fixed Output) (continued)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

B. To simplify the capacitor selection procedure, refer to the quick B. From the quick design component selection table shown in

design component selection table shown in Figure 28. This table Figure 28, locate the 5V output voltage section. In the load current

contains different input voltages, output voltages, and load currents, column, choose the load current line that is closest to the current

and lists various inductors and output capacitors that will provide the needed in your application, for this example, use the 1A line. In the

best design solutions.

maximum input voltage column, select the line that covers the input

voltage needed in your application, in this example, use the 15V line.

Continuing on this line are recommended inductors and capacitors

that will provide the best overall performance.

The capacitor list contains both through hole electrolytic and surface

mount

tantalum

capacitors

from

four

different

capacitor

manufacturers. It is recommended that both the manufacturers and

the manufacturer's series that are listed in the table be used.

In this example aluminum electrolytic capacitors from several

different manufacturers are available with the range of ESR numbers

needed.

220

μF

25V

Panasonic HFQ Series

220

μF

25V

Nichicon PL Series

C. The capacitor voltage rating for electrolytic capacitors should be C. For a 5V output, a capacitor voltage rating at least 7.5V or more

at least 1.5 times greater than the output voltage, and often much is needed. But, in this example, even a low ESR, switching grade,

higher voltage ratings

are

needed

to satisfy the

low

ESR 220

μF

10V

aluminum

electrolytic

capacitor

would

exhibit

requirements for low output ripple voltage

approximately 225 m

Ω of ESR (see the curve in Figure 34 for the

.

ESR vs voltage rating). This amount of ESR would result in relatively

high output ripple voltage. To reduce the ripple to 1% of the output

voltage, or less, a capacitor with a higher voltage rating (lower ESR)

should be selected. A 16V or 25V capacitor will reduce the ripple

voltage by approximately half.

D. For computer aided design software, see Switchers Made

Simple™ (version 4.2 or later).

3. Catch Diode Selection (D1)

3. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times greater A. Refer to the table shown in Table 8. In this example, a 3A, 20V,

than the maximum load current. Also, if the power supply design 1N5820 Schottky diode will provide the best performance, and will

must withstand a continuous output short, the diode should have a not be overstressed even for a shorted output.

current rating equal to the maximum current limit of the LM2598. The

most stressful condition for this diode is an overload or shorted

output condition.

B. The reverse voltage rating of the diode should be at least 1.25

times the maximum input voltage.

C. This diode must be fast (short reverse recovery time) and must be

located close to the LM2598 using short leads and short printed

circuit traces. Because of their fast switching speed and low forward

voltage drop, Schottky diodes provide the best performance and

efficiency, and should be the first choice, especially in low output

voltage applications. Ultra-fast recovery, or High-Efficiency rectifiers

also provide good results. Ultra-fast recovery diodes typically have

reverse recovery times of 50 ns or less. Rectifiers such as the

1N5400 series are much too slow and should not be used.

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