LM1575/LM2575/LM2575HV
LM2575 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
Figures 3, 4, 5, 6
(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 inductance region intersected by V
IN
(Max) and
I
LOAD
(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
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 x I
LOAD
. For additional inductor
information, see the inductor 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 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) A.
The catch-diode current
rating must be at least 1.2 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 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 (C
IN
)
An aluminum or tantalum
electrolytic bypass capacitor located close to the regulator is
needed for stable operation.
EXAMPLE (Fixed Output Voltage Versions)
Given:
V
OUT
= 5V V
IN
(Max) = 20V I
LOAD
(Max) = 0.8A
1. Inductor Selection (L1) A.
Use the selection guide
shown in
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
choose AIE 415-0926, Pulse Engineering
PE-52627, or RL1952.
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.
For this example, a 1A
current rating is adequate.
B.
Use a 30V 1N5818 or SR103
Schottky diode, or any of the suggested fast-recovery
diodes shown in
4. Input Capacitor (C
IN
)
A 47 µF, 25V aluminum electrolytic
capacitor located near the input and ground pins provides
sufficient bypassing.
13
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