LM2576/LM2576HV
LM2576 Series Buck Regulator Design Procedure
PROCEDURE (Adjustable Output Voltage Versions)
2. Inductor Selection (L1) A.
Calculate the inductor Volt
•
microsecond constant, E
•
T (V
•
µs), from the
following formula:
(Continued)
EXAMPLE (Adjustable Output Voltage Versions)
2. Inductor Selection (L1) A.
Calculate E
•
T (V
•
µs)
B.
Use the E
•
T value from the previous formula and
match it with the E
•
T number on the vertical axis of the
Inductor Value Selection Guide
shown in
C.
On
the horizontal axis, select the maximum load current.
D.
Identify the inductance region intersected by the E
•
T
value and the maximum load current value, and note the
inductor code for that region.
E.
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 fre-
quency (52 kHz) and for a current rating of 1.15 x I
LOAD
.
For additional inductor information, see the inductor sec-
tion in the application hints section of this data sheet.
3. 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, the capacitor must satisfy the
following requirement:
B.
E
•
T = 115 V
•
µs
C.
I
LOAD
(Max) = 3A
D.
Inductance
Region = H150
E.
Inductor Value = 150 µH
Choose from
AIE
part #415-0936
Pulse Engineering
part
#PE-531115,
or
Renco part #RL2445.
3. Output Capacitor Selection (C
OUT
)
However, for acceptable output ripple voltage select C
OUT
≥
680 µF C
OUT
= 680 µF electrolytic capacitor
The above formula yields capacitor values between 10 µF
and 2200 µF that will satisfy the loop requirements for
stable operation. But to achieve an acceptable output
ripple voltage, (approximately 1% of the output voltage)
and transient response, the output capacitor may need to
be several times larger than the above formula yields.
B.
The capacitor’s voltage rating should be at last 1.5 times
greater than the output voltage. For a 10V regulator, a
rating of at least 15V or more is recommended. Higher
voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to select
a capacitor rate for a higher voltage than would normally be
needed.
4. 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. See diode
selection guide in
B.
The reverse voltage rating
of the diode should be at least 1.25 times the maximum
input voltage.
5. Input Capacitor (C
IN
)
An aluminum or tantalum
electrolytic bypass capacitor located close to the
regulator is needed for stable operation.
To further simplify the buck regulator design procedure, Na-
tional Semiconductor is making available computer design
software to be used with the SIMPLE SWITCHER line of
4. Catch Diode Selection (D1) A.
For this example, a
3.3A current rating is adequate.
B.
Use a 30V 31DQ03
Schottky diode, or any of the suggested fast-recovery
diodes in
5. Input Capacitor (C
IN
)
A 100 µF aluminum electrolytic
capacitor located near the input and ground pins
provides sufficient bypassing.
switching regulators.
Switchers Made Simple
(Version 3.3)
is available on a (3
1
⁄
2
") diskette for IBM compatible comput-
ers from a National Semiconductor sales office in your area.
www.national.com
14
NSC [ NATIONAL SEMICONDUCTOR ]
