SNVS124C – NOVEMBER 1999 – REVISED APRIL 2013
LM2596 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version)
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)
A.
Select the correct inductor value selection guide from Figures
or
(Output voltages of 3.3V, 5V, or
12V respectively.) For all other voltages, see the
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).
Given:
V
OUT
= 5V
V
IN
(max) = 12V
I
LOAD
(max) = 3A
1. Inductor Selection (L1)
A.
Use the inductor selection guide for the 5V version shown in
B.
From the inductor value selection guide shown in
the
inductance region intersected by the 12V horizontal line and the 3A
vertical line is 33
μH,
and the inductor code is L40.
EXAMPLE (Fixed Output Voltage Version)
C.
The inductance value required is 33
μH.
From the table in
go to the L40 line and choose an inductor part number from
any of the four manufacturers shown. (In most instance, both
C.
Select an appropriate inductor from the four manufacturer's part through hole and surface mount inductors are available.)
numbers listed in
2. Output Capacitor Selection (C
OUT
)
A. See section on output capacitors in
section.
B.
From the quick design component selection table shown in
locate the 5V output voltage section. In the load current
column, choose the load current line that is closest to the current
needed in your application, for this example, use the 3A line. In the
maximum input voltage column, select the line that covers the input
voltage needed in your application, in this example, use the 15V line.
B.
To simplify the capacitor selection procedure, refer to the quick Continuing on this line are recommended inductors and capacitors
design component selection table shown in
This table that will provide the best overall performance.
contains different input voltages, output voltages, and load currents, The capacitor list contains both through hole electrolytic and surface
and lists various inductors and output capacitors that will provide the mount tantalum capacitors from four different capacitor
best design solutions.
manufacturers. It is recommended that both the manufacturers and
C.
The capacitor voltage rating for electrolytic capacitors should be the manufacturer's series that are listed in the table be used.
at least 1.5 times greater than the output voltage, and often much In this example aluminum electrolytic capacitors from several
higher voltage ratings are needed to satisfy the low ESR different manufacturers are available with the range of ESR numbers
requirements for low output ripple voltage.
needed.
D.
For computer aided design software, see Switchers Made
330
μF
35V Panasonic HFQ Series
Simple™ version 4.3 or later.
330
μF
35V Nichicon PL Series
C.
For a 5V output, a capacitor voltage rating at least 7.5V or more
is needed. But even a low ESR, switching grade, 220
μF
10V
aluminum electrolytic capacitor would exhibit approximately 225 mΩ
of ESR (see the curve in
for the ESR vs voltage rating).
This amount of ESR would result in relatively high output ripple
voltage. To reduce the ripple to 1% of the output voltage, or less, a
capacitor with a higher value or with a higher voltage rating (lower
ESR) should be selected. A 16V or 25V capacitor will reduce the
ripple voltage by approximately half.
2. Output Capacitor Selection (C
OUT
)
A.
In the majority of applications, low ESR (Equivalent Series
Resistance) electrolytic capacitors between 82
μF
and 820
μF
and
low ESR solid tantalum capacitors between 10
μF
and 470
μF
provide the best results. This capacitor should be located close to
the IC using short capacitor leads and short copper traces. Do not
use capacitors larger than 820
μF
.
For additional information, see section on output capacitors in
section.
10
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