LM1575, LM2575-N, LM2575HV
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SNVS106E –MAY 1999–REVISED APRIL 2013
LM2575 Series Buck Regulator Design Procedure
PROCEDURE (Fixed Output Voltage Versions)
EXAMPLE (Fixed Output Voltage Versions)
Given:
Given:
VOUT = 5V
VIN(Max) = Maximum Input Voltage
VIN(Max) = 20V
VOUT = Regulated Output Voltage (3.3V, 5V, 12V, or 15V)
ILOAD(Max) = Maximum Load Current
ILOAD(Max) = 0.8A
1. Inductor Selection (L1)
1. Inductor Selection (L1)
A. Select the correct Inductor value selection guide from Figure 27, A. Use the selection guide shown in Figure 28.
Figure 28, Figure 29 and Figure 30 (Output voltages of 3.3V, 5V,
12V or 15V respectively). For other output voltages, see the design
procedure of Figure 26 .
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 Table 2,
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
inductor code for that region.
C. Identify the inductor value from the inductor code, and select an
appropriate inductor from the table shown in Table 2. 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 INDUCTOR SELECTION.
2. Output Capacitor Selection (COUT
)
2. Output Capacitor Selection (COUT)
A. The value of the output capacitor together with the inductor A. COUT = 100 μF to 470 μF standard aluminum electrolytic.
defines the dominate pole-pair of the switching regulator loop. For
B. Capacitor voltage rating = 20V.
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 recommended.
Higher voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to select 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 1.2 times greater A. For this example, a 1A current rating is adequate.
than the maximum load current. Also, if the power supply design
B. Use a 30V 1N5818 or SR103 Schottky diode, or any of the
must withstand a continuous output short, the diode should have a
suggested fast-recovery diodes shown in Table 1.
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. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
4. Input Capacitor (CIN
An aluminum or tantalum electrolytic bypass capacitor located close A 47 μF, 25V aluminum electrolytic capacitor located near the input
to the regulator is needed for stable operation. and ground pins provides sufficient bypassing.
)
4. Input Capacitor (CIN)
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