Datasheet

LM2597, LM2597HV
SNVS119C –MARCH 1998–REVISED APRIL 2013 www.ti.com
LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given: Given:
V
OUT
= Regulated Output Voltage (3.3V, 5V or 12V) V
OUT
= 5V
V
IN
(max) = Maximum DC Input Voltage V
IN
(max) = 12V
I
LOAD
(max) = Maximum Load Current I
LOAD
(max) = 0.4A
1. Inductor Selection (L1) 1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from Figure 26, A. Use the inductor selection guide for the 5V version shown in
Figure 27, or Figure 28. (Output voltages of 3.3V, 5V, or 12V Figure 27.
respectively.) For all other voltages, see the design procedure for the
B. From the inductor value selection guide shown in Figure 27, the
adjustable version.
inductance region intersected by the 12V horizontal line and the 0.4A
B. From the inductor value selection guide, identify the inductance vertical line is 100 μH, and the inductor code is L20.
region intersected by the Maximum Input Voltage line and the
C. The inductance value required is 100 μH. From the table in
Maximum Load Current line. Each region is identified by an
Table 3, go to the L20 line and choose an inductor part number from
inductance value and an inductor code (LXX).
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 Table 3.
2. Output Capacitor Selection (C
OUT
) 2. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR (Equivalent Series A. See OUTPUT CAPACITOR in Application Information section.
Resistance) electrolytic capacitors between 82 μF and 220 μF and
B. From the quick design component selection table shown in
low ESR solid tantalum capacitors between 15 μF and 100 μF
Table 1, locate the 5V output voltage section. In the load current
provide the best results. This capacitor should be located close to
column, choose the load current line that is closest to the current
the IC using short capacitor leads and short copper traces. Do not
needed in your application, for this example, use the 0.5A line. In the
use capacitors larger than 220 μF.
maximum input voltage column, select the line that covers the input
For additional information, see OUTPUT CAPACITOR in voltage needed in your application, in this example, use the 15V line.
Application Information. Continuing on this line are recommended inductors and capacitors
that will provide the best overall performance.
B. To simplify the capacitor selection procedure, refer to the quick
design component selection table shown in Table 1. This table The capacitor list contains both through hole electrolytic and surface
contains different input voltages, output voltages, and load currents, mount tantalum capacitors from four different capacitor
and lists various inductors and output capacitors that will provide the manufacturers. It is recommended that both the manufacturers and
best design solutions. the manufacturer's series that are listed in the table be used.
C. The capacitor voltage rating for electrolytic capacitors should be In this example aluminum electrolytic capacitors from several
at least 1.5 times greater than the output voltage, and often much different manufacturers are available with the range of ESR numbers
higher voltage ratings are needed to satisfy the low ESR needed.
requirements for low output ripple voltage.
120 μF 25V Panasonic HFQ Series
D. For computer aided design software, see Switchers Made
120 μF 25V Nichicon PL Series
Simple
®
version 4.1 or later).
C. For a 5V output, a capacitor voltage rating at least 7.5V or more
is needed. But, in this example, even a low ESR, switching grade,
120 μF 10V aluminum electrolytic capacitor would exhibit
approximately 400 mΩ of ESR (see the curve in Figure 36 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 voltage rating (lower ESR)
should be selected. A 16V or 25V capacitor will reduce the ripple
voltage by approximately half.
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