LM1575 National Semiconductor, LM1575 Datasheet - Page 11

• Manufacturer Part Number: LM1575
• Description: SIMPLE SWITCHER 1A Step-Down Voltage Regulator
• Manufacturer: National Semiconductor

LM2575 Series Buck Regulator Design Procedure
Given:
V
V
I
1. Inductor Selection (L1)
A. Select the correct Inductor value selection guide from Fig-
ures 3, 4, 5, 6 (Output voltages of 3.3V, 5V, 12V or 15V re-
spectively). For other output voltages, see the design proce-
dure for the adjustable version.
B. From the inductor value selection guide, identify the in-
ductance region intersected by V
and note the inductor code for that region.
C. Identify the inductor value from the inductor code, and se-
lect 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 rat-
ing of 1.15 x I
the inductor section in the Application Hints section of this
data sheet.
2. Output Capacitor Selection (C
A. The value of the output capacitor together with the induc-
tor 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 be-
tween 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 rec-
ommended.
Higher voltage electrolytic capacitors generally have lower
ESR numbers, and for this reasion it may be necessary to
select a capacitor rated for a higher voltage than would nor-
mally 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
An aluminum or tantalum electrolytic bypass capacitor lo-
cated close to the regulator is needed for stable operation.
LOAD
OUT
IN
(Max) = Maximum Input Voltage
PROCEDURE (Fixed Output Voltage Versions)
(Max) = Maximum Load Current
= Regulated Output Voltage (3.3V, 5V, 12V, or 15V)
LOAD
. For additional inductor information, see
IN
)
IN
OUT
(Max) and I
)
LOAD
(Max),
11
Given:
V
V
I
1. Inductor Selection (L1)
A. Use the selection guide shown in Figure 4 .
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 Fig-
ure 9 , choose AIE 415-0926, Pulse Engineering PE-52627,
or RL1952.
2. Output Capacitor Selection (C
A. C
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 Figure 8 .
4. Input Capacitor (C
A 47 µF, 25V aluminum electrolytic capacitor located near
the input and ground pins provides sufficient bypassing.
LOAD
OUT
IN
(Max) = 20V
OUT
(Max) = 0.8A
= 5V
EXAMPLE (Fixed Output Voltage Versions)
= 100 µF to 470 µF standard aluminum electrolytic.
IN
)
OUT
)
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