LM2596T-3.3 National Semiconductor, LM2596T-3.3 Datasheet - Page 10



Manufacturer Part Number
National Semiconductor

Specifications of LM2596T-3.3

Rohs Compliant
LM2596 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version)
= Regulated Output Voltage (3.3V, 5V or 12V)
(max) = Maximum DC Input Voltage
(max) = Maximum Load Current
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from Fig-
ures Figure 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.
B. From the inductor value selection guide, identify the induc-
tance 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 .
2. Output Capacitor Selection (C
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 capaci-
tors in application information section.
B. To simplify the capacitor selection procedure, refer to the
quick design component selection table shown in Figure 2 .
This table contains different input voltages, output voltages,
and load currents, and lists various inductors and output ca-
pacitors that will provide the best design solutions.
C. The capacitor voltage rating for electrolytic capacitors
should be at least 1.5 times greater than the output voltage,
and often much higher voltage ratings are needed to satisfy
the low ESR requirements for low output ripple voltage.
D. For computer aided design software, see Switchers Made
version 4.3 or later.
EXAMPLE (Fixed Output Voltage Version)
= 5V
(max) = 12V
(max) = 3A
1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version shown
in Figure 5 .
B. From the inductor value selection guide shown in Figure 5 ,
the inductance region intersected by the 12V horizontal line
and the 3A vertical line is 33 µH, and the inductor code is L40.
C. The inductance value required is 33 µH. From the table in
Figure 8 , go to the L40 line and choose an inductor part
number from any of the four manufacturers shown. (In most
instance, both through hole and surface mount inductors are
2. Output Capacitor Selection (C
A. See section on output capacitors in application infor-
mation section.
B. From the quick design component selection table shown in
Figure 2 , 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. Continuing on this line are
recommended inductors and capacitors that will provide the
best overall performance.
The capacitor list contains both through hole electrolytic and
surface mount tantalum capacitors from four different capaci-
tor manufacturers. It is recommended that both the manufac-
turers and the manufacturer’s series that are listed in the table
be used.
In this example aluminum electrolytic capacitors from several
different manufacturers are available with the range of ESR
numbers needed.
330 µF 35V Panasonic HFQ Series
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 approxi-
mately 225 m of ESR (see the curve in Figure 14 for the ESR
vs voltage rating). This amount of ESR would result in rela-
tively 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 approxi-
mately half.

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