LM2575 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions)
EXAMPLE (Fixed Output Voltage Versions)
Given:
Given:
VOUT = Regulated Output Voltage (3.3V, 5V, 12V, or 15V)
VIN(Max) = Maximum Input Voltage
VOUT = 5V
VIN(Max) = 20V
ILOAD(Max) = 0.8A
1. Inductor Selection (L1)
ILOAD(Max) = Maximum Load Current
1. Inductor Selection (L1)
A. Select the correct Inductor value selection guide from Figures A. Use the selection guide shown in Figure 4.
3, 4, 5, 6 (Output voltages of 3.3V, 5V, 12V or 15V respectively).
For other output voltages, see the design procedure for the ad-
justable version.
B. From the selection guide, the inductance area intersected by
the 20V line and 0.8A line is L330.
C. Inductor value required is 330 μH. From the table in Figure 9,
choose AIE 415-0926, Pulse Engineering PE-52627, or RL1952.
B. From the inductor value selection guide, identify the inductance
region intersected by VIN(Max) and ILOAD(Max), and note the in-
ductor code for that region.
C. Identify the inductor value from the inductor code, and select an
appropriate inductor from the table shown in Figure 9. Part numbers
are listed for three inductor manufacturers. The inductor chosen
must be rated for operation at the LM2575 switching frequency (52
kHz) and for a current rating of 1.15 × ILOAD. For additional inductor
information, see the inductor section in the Application Hints section
of this data sheet.
2. Output Capacitor Selection (COUT
)
2. Output Capacitor Selection (COUT)
A. The value of the output capacitor together with the inductor de- A. COUT = 100 μF to 470 μF standard aluminum electrolytic.
fines the dominate pole-pair of the switching regulator loop. For
stable operation and an acceptable output ripple voltage, (approx-
imately 1% of the output voltage) a value between 100 μF and 470
μF is recommended.
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 recommended.
Higher voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to select a ca-
pacitor 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 1.2 times greater A. For this example, a 1A current rating is adequate.
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 LM2575.
The most stressful condition for this diode is an overload or shorted
output condition.
B. Use a 30V 1N5818 or SR103 Schottky diode, or any of the
suggested fast-recovery diodes shown in Figure 8.
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 A 47 μF, 25V aluminum electrolytic capacitor located near the input
close to the regulator is needed for stable operation. and ground pins provides sufficient bypassing.
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