LM2598 datasheet(12/33 Pages) NSC

12 / 33 page

LM2598 Series Buck Regulator Design Procedure (Fixed Output) (Continued)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

4. Input Capacitor (C

IN)

A low ESR aluminum or tantalum bypass capacitor is needed

between the input pin and ground to prevent large voltage

transients from appearing at the input. In addition, the RMS

current rating of the input capacitor should be selected to be

at least 1⁄2 the DC load current. The capacitor manufacturers

data sheet must be checked to assure that this current rating

is not exceeded. The curve shown in

Figure 16 shows typical

RMS current ratings for several different aluminum electro-

lytic capacitor values.

This capacitor should be located close to the IC using short

leads and the voltage rating should be approximately 1.5

times the maximum input voltage.

If solid tantalum input capacitors are used, it is recomended

that they be surge current tested by the manufacturer.

Use caution when using ceramic capacitors for input bypass-

ing, because it may cause severe ringing at the V

IN pin.

For additional information, see section on input capaci-

tors in Application Information section.

4. Input Capacitor (C

IN)

The important parameters for the Input capacitor are the

input voltage rating and the RMS current rating. With a

nominal input voltage of 12V, an aluminum electrolytic ca-

pacitor with a voltage rating greater than 18V (1.5 x V

IN)

would be needed. The next higher capacitor voltage rating is

25V.

The RMS current rating requirement for the input capacitor in

a buck regulator is approximately 1⁄2 the DC load current. In

this example, with a 1A load, a capacitor with a RMS current

rating of at least 500 mA is needed. The curves shown in

Figure 16 can be used to select an appropriate input capaci-

tor. From the curves, locate the 25V line and note which

capacitor values have RMS current ratings greater than 500

mA. Either a 180 µF or 220 µF, 25V capacitor could be used.

For a through hole design, a 220 µF/25V electrolytic capaci-

tor (Panasonic HFQ series or Nichicon PL series or equiva-

lent) would be adequate. other types or other manufacturers

capacitors can be used provided the RMS ripple current

ratings are adequate.

For surface mount designs, solid tantalum capacitors are

recommended. The TPS series available from AVX, and the

593D series from Sprague are both surge current tested.

Conditions

Inductor

Output Capacitor

Through Hole Electrolytic

Surface Mount Tantalum

Output

Load

Max Input

Inductance

Inductor

Panasonic

Nichicon

AVX TPS

Sprague

Voltage

Current

Voltage

(µH)

(#)

HFQ Series

PL Series

Series

595D Series

(V)

(A)

(V)

(µF/V)

3.3

L24

330/16

220/10

330/10

L23

270/25

220/10

270/10

L31

220/25

220/35

220/10

L30

180/35

220/35

220/10

180/10

L13

220/25

220/16

220/10

0.5

L21

150/35

150/25

100/16

150/16

100

L20

150/35

82/35

100/16

100/20

L28

330/16

220/10

270/10

L31

220/25

220/10

L30

180/35

220/10

150/16

100

L29

180/35

120/35

100/16

120/16

L21

180/16

220/10

150/16

0.5

150

L19

120/25

100/16

100/20

150

L19

100/25

68/20

68/25

L31

220/25

68/20

120/20

L30

180/35

120/25

68/20

120/20

150

L36

82/25

68/20

100/20

220

L35

82/25

68/20

68/25

L21

180/25

68/20

120/20

0.5

150

L19

82/25

68/20

100/20

330

L26

56/25

68/20

68/25

FIGURE 2. LM2598 Fixed Voltage Quick Design Component Selection Table

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