LM2594 datasheet(14/28 Pages) NSC

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LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable

Output) (Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

6. 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 13 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.

6. Input Capacitor (C

IN)

The important parameters for the Input capacitor are the in-

put voltage rating and the RMS current rating. With a nominal

input voltage of 28V, an aluminum electrolytic aluminum elec-

trolytic capacitor with a voltage rating greater than 42V (1.5 x

IN) would be needed. Since the the next higher capacitor

voltage rating is 50V, a 50V capacitor should be used. The

capacitor voltage rating of (1.5 x V

IN) is a conservative guide-

line, and can be modified somewhat if desired.

The RMS current rating requirement for the input capacitor of

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

this example, with a 400 mA load, a capacitor with a RMS

current rating of at least 200 mA is needed.

The curves shown in

Figure 13 can be used to select an ap-

propriate input capacitor. From the curves, locate the 50V

line and note which capacitor values have RMS current rat-

ings greater than 200 mA. A 47 µF/50V low ESR electrolytic

capacitor capacitor is needed.

For a through hole design, a 47 µF/50V electrolytic capacitor

(Panasonic HFQ series or Nichicon PL series or equivalent)

would be adequate. Other types or other manufacturers ca-

pacitors 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.

To further simplify the buck regulator design procedure, Na-

tional Semiconductor is making available computer design

software to be used with the Simple Switcher line ot switch-

ing regulators. Switchers Made Simple (version 4.1 or later)

is available from National’s web site, www.national.com.

Output

Volt-

age

(V)

Through Hole Output Capacitor

Surface Mount Output Capacitor

Panasonic

Nichicon PL

Feedforward

AVX TPS

Sprague

Feedforward

HFQ Series

Series

Capacitor

Series

595D Series

Capacitor

(µF/V)

1.2

220/25

220/10

180/25

4.7 nF

100/10

120/10

4.7 nF

82/25

4.7 nF

100/10

120/10

4.7 nF

82/25

3.3 nF

100/16

3.3 nF

82/25

2.2 nF

100/16

2.2 nF

82/25

1.5 nF

68/20

100/20

1.5 nF

82/50

120/50

1 nF

10/35

15/35

220 pF

82/50

120/50

820 pF

10/35

15/35

220 pF

FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table

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