LM2674 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version)
To simplify the buck regulator design procedure, National
Semiconductor is making available computer design software to
be used with the SIMPLE SWITCHER line of switching
regulators. LM267X Made Simple (version 6.0)is available on
Windows® 3.1, NT, or 95 operating systems.
EXAMPLE (Fixed Output Voltage Version)
Given:
Given:
VOUT = 5V
VOUT = Regulated Output Voltage (3.3V, 5V, or 12V)
V
IN(max) = Maximum DC Input Voltage
LOAD(max) = Maximum Load Current
1. Inductor Selection (L1)
VIN(max) = 12V
I
ILOAD(max) = 500 mA
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from Figure A. Use the inductor selection guide for the 5V version shown in
4, Figure 5 or Figure 6 (output voltages of 3.3V, 5V, or 12V
respectively). For all other voltages, see the design procedure
for the adjustable version.
Figure 5.
B. From the inductor value selection guide, identify the
inductance region intersected by the Maximum Input Voltage
line and the Maximum Load Current line. Each region is
identified by an inductance value and an inductor code (LXX).
C. Select an appropriate inductor from the four manufacturer’s
part numbers listed in Figure 8. Each manufacturer makes a
different style of inductor to allow flexibility in meeting various
design requirements. Listed below are some of the
differentiating characteristics of each manufacturer’s inductors:
Schott: ferrite EP core inductors; these have very low leakage
magnetic fields to reduce electro-magnetic interference (EMI)
and are the lowest power loss inductors
B. From the inductor value selection guide shown in Figure 5,
the inductance region intersected by the 12V horizontal line and
the 500mA vertical line is 47 µH, and the inductor code is L13.
C. The inductance value required is 47 µH. From the table in
Figure 8, go to the L13 line and choose an inductor part number
from any of the four manufacturers shown. (In most instances,
both through hole and surface mount inductors are available.)
Renco: ferrite stick core inductors; benefits are typically lowest
cost inductors and can withstand E•T and transient peak
currents above rated value. Be aware that these inductors have
an external magnetic field which may generate more EMI than
other types of inductors.
Pulse: powered iron toroid core inductors; these can also be low
cost and can withstand larger than normal E•T and transient
peak currents. Toroid inductors have low EMI.
Coilcraft: ferrite drum core inductors; these are the smallest
physical size inductors, available only as SMT components. Be
aware that these inductors also generate EMI—but less than
stick inductors.
Complete specifications for these inductors are available from
the respective manufacturers. A table listing the manufacturers’
phone numbers is located in Figure 9.
2. Output Capacitor Selection (COUT
)
2. Output Capacitor Selection (COUT)
A. Select an output capacitor from the output capacitor table in A. Use the 5.0V section in the output capacitor table in Figure
Figure 10. Using the output voltage and the inductance value
found in the inductor selection guide, step 1, locate the
appropriate capacitor value and voltage rating.
10. Choose a capacitor value and voltage rating from the line
that contains the inductance value of 47 µH. The capacitance
and voltage rating values corresponding to the 47 µH inductor
are the:
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