LM2576/LM2576HV
LM2576 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
or
(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 LM2576 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 LM2576. 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) = 15V I
LOAD
(Max) = 3A
1. Inductor Selection (L1) A.
Use the selection guide
shown in
B.
From the selection guide, the
inductance area intersected by the 15V line and 3A line
is L100.
C.
Inductor value required is 100 µH. From the
table in
Choose AIE 415-0930, Pulse
Engineering PE92108, or Renco RL2444.
2. Output Capacitor Selection (C
OUT
) A.
C
OUT
= 680
µF to 2000 µF standard aluminum electrolytic.
B.Capacitor
voltage rating = 20V.
3. Catch Diode Selection (D1) A.For
this example, a 3A
current rating is adequate.
B.
Use a 20V 1N5823 or
SR302 Schottky diode, or any of the suggested
fast-recovery diodes shown in
4. Input Capacitor (C
IN
)
A 100 µF, 25V aluminum
electrolytic capacitor located near the input and ground
pins provides sufficient bypassing.
11
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
