SNVS129E – MAY 2004 – REVISED JUNE 2005
LM2675 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version)
To simplify the buck regulator design procedure, Texas Instruments
is making available computer design software to be used with the
SIMPLE SWITCHERline
of switching regulators.LM267X
Made
Simpleversion
6.0 is available on
Windows
®
3.1, NT, or 95 operating
systems.
Given:
V
OUT
= Regulated Output Voltage (3.3V, 5V, or 12V)
V
IN
(max) = Maximum DC Input Voltage
I
LOAD
(max) = Maximum Load Current
1. Inductor Selection (L1)
Given:
V
OUT
= 5V
V
IN
(max) = 12V
I
LOAD
(max) = 1A
1. Inductor Selection (L1)
EXAMPLE (Fixed Output Voltage Version)
A.
Select the correct inductor value selection guide from
A.
Use the inductor selection guide for the 5V version shown in
or
(output voltages of 3.3V, 5V, or 12V
respectively). For all other voltages, see the design procedure for the
adjustable version.
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
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
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
2. Output Capacitor Selection (C
OUT
)
A.
Select an output capacitor from the output capacitor table in
Using the output voltage and the inductance value found in
the inductor selection guide, step 1, locate the appropriate capacitor
value and voltage rating.
The capacitor list contains through-hole electrolytic capacitors from
four different capacitor manufacturers and surface mount tantalum
capacitors from two different capacitor manufacturers. It is
recommended that both the manufacturers and the manufacturer's
series that are listed in the table be used. A table listing the
manufacturers' phone numbers is located in
2. Output Capacitor Selection (C
OUT
)
A.
Use the 5.0V section in the output capacitor table in
Choose a capacitor value and voltage rating from the line that
contains the inductance value of 33
μH.
The capacitance and
voltage rating values corresponding to the 33
μH
inductor are the:
Surface Mount:
68
μF/10V
Sprague 594D Series.
100
μF/10V
AVX TPS Series.
Through Hole:
68
μF/10V
Sanyo OS-CON SA Series.
220
μF/35V
Sanyo MV-GX Series.
220
μF/35V
Nichicon PL Series.
220
μF/35V
Panasonic HFQ Series.
B.
From the inductor value selection guide shown in
the
inductance region intersected by the 12V horizontal line and the 1A
vertical line is 33
μH,
and the inductor code is L23.
C.
The inductance value required is 33
μH.
From the table in
go to the L23 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.)
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