LM2574 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions)
Given:
V
OUT
= Regulated Output Voltage (3.3V, 5V, 12V, or 15V)
V
IN
(Max) = Maximum Input Voltage
I
LOAD
(Max) = Maximum Load Current
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 I
LOAD
(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.
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.
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.
4. Input Capacitor (C
IN
)
An aluminum or tantalum electrolytic bypass capacitor located
close to the regulator is needed for stable operation.
2. Output Capacitor Selection (C
OUT
)
A.
C
OUT
= 100 µF to 470 µF standard aluminum electrolytic.
B.
Capacitor voltage rating = 20V.
EXAMPLE (Fixed Output Voltage Versions)
Given:
V
OUT
= 5V
V
IN
(Max) = 15V
I
LOAD
(Max) = 0.4A
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.
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
)
A 22 µF aluminum electrolytic capacitor located near the input
and ground pins provides sufficient bypassing.
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