LM2574 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions)
EXAMPLE (Fixed Output Voltage Versions)
Given:
Given:
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VOUT 5V
VOUT Regulated Output Voltage (3.3V, 5V, 12V, or 15V)
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VIN(Max) 15V
VIN(Max) Maximum Input Voltage
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ILOAD(Max) Maximum Load Current
ILOAD(Max) 0.4A
1. Inductor Selection (L1)
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.
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.
B. From the inductor value selection guide, identify the induc-
tance region intersected by VIN(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 ILOAD. For additional inductor information, see the in-
ductor section in the Application Hints section of this data
sheet.
2. Output Capacitor Selection (COUT
)
2. Output Capacitor Selection (COUT)
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A. COUT 100 µF to 470 µF standard aluminum electrolytic.
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.
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B. Capacitor voltage rating 20V.
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.
3. Catch Diode Selection (D1)
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.
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.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
4. Input Capacitor (CIN
)
4. Input Capacitor (CIN)
An aluminum or tantalum electrolytic bypass capacitor located
close to the regulator is needed for stable operation.
A 22 µF aluminum electrolytic capacitor located near the input
and ground pins provides sufficient bypassing.
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