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LM2574 Series Buck Regulator Design Procedure

PROCEDURE (Fixed Output Voltage Versions)

EXAMPLE (Fixed Output Voltage Versions)

Given:

OUT = Regulated Output Voltage (3.3V, 5V, 12V, or 15V)

IN(Max) = Maximum Input Voltage

LOAD(Max) = Maximum Load Current

Given:

OUT = 5V

IN(Max) = 15V

LOAD(Max) = 0.4A

1. Inductor Selection (L1)

A. Select the correct Inductor value selection guide from

Fig-

ures 4, 5, 6,or Figure 7. (Output voltages of 3.3V, 5V, 12V or

15V respectively). For other output voltages, see the design

procedure for the adjustable version.

B. From the inductor value selection guide, identify the induc-

tance region intersected by V

IN(Max) and ILOAD(Max).

C. Select an appropriate inductor from the table shown in

Fig-

ure 3. Part numbers are listed for three inductor manufactur-

ers. The inductor chosen must be rated for operation at the

LM2574 switching frequency (52 kHz) and for a current rating

of 1.5 x I

LOAD. For additional inductor information, see the in-

ductor section in the Application Hints section of this data

sheet.

1. Inductor Selection (L1)

A. Use the selection guide shown in

Figure 5.

B. From the selection guide, the inductance area intersected

by the 15V line and 0.4A line is 330.

C. Inductor value required is 330 µH. From the table in

Figure

3, choose Pulse Engineering PE-52627, Renco RL-1284-330,

or NPI NP5920/5921.

2. Output Capacitor Selection (C

OUT)

A. The value of the output capacitor together with the inductor

defines the dominate pole-pair of the switching regulator loop.

For stable operation and an acceptable output ripple voltage,

(approximately 1% of the output voltage) a value between

100 µF and 470 µF is recommended.

B. The capacitor’s voltage rating should be at least 1.5 times

greater than the output voltage. For a 5V regulator, a rating of

at least 8V is appropriate, and a 10V or 15V rating is recom-

mended.

Higher voltage electrolytic capacitors generally have lower

ESR numbers, and for this reason it may be necessary to se-

lect a capacitor rated for a higher voltage than would normally

be needed.

2. Output Capacitor Selection (C

OUT)

A. C

OUT = 100 µF to 470 µF standard aluminum electrolytic.

B. Capacitor voltage rating = 20V.

3. Catch Diode Selection (D1)

A. The catch-diode current rating must be at least 1.5 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 cur-

rent limit of the LM2574. 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.

3. Catch Diode Selection (D1)

A. For this example, a 1A current rating is adequate.

B. Use a 20V 1N5817 or SR102 Schottky diode, or any of the

suggested fast-recovery diodes shown in

Figure 9.

4. Input Capacitor (C

IN)

An aluminum or tantalum electrolytic bypass capacitor located

close to the regulator is needed for stable operation.

4. Input Capacitor (C

IN)

A 22 µF aluminum electrolytic capacitor located near the input

and ground pins provides sufficient bypassing.

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