LM2597 datasheet(14/34 Pages) NSC

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LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable

Output) (Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

5. Catch Diode Selection (D1)

A. The catch diode current rating must be at least 1.3 times

greater than the maximum load current. Also, if the power

supply design must withstand a continuous output short, the

diode should have a current rating equal to the maximum

current limit of the LM2597. 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 LM2597 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 are also a good choice,

but some types with an abrupt turn-off characteristic may

cause instability or EMl problems. Ultra-fast recovery diodes

typically have reverse recovery times of 50 ns or less. Recti-

fiers such as the 1N4001 series are much too slow and should

not be used.

5. Catch Diode Selection (D1)

A. Refer to the table shown in

Figure 10. Schottky diodes

provide the best performance, and in this example a 1A, 40V,

1N5819 Schottky diode would be a good choice. The 1A diode

rating is more than adequate and will not be overstressed

even for a shorted output.

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 16 shows typical

RMS current ratings for several different aluminum electrolytic

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 capacitor

in application information section.

6. 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 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 16 can be used to select an

appropriate 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 switching

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

available at National’s web site, www.national.com.

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