Datasheet

LM2594, LM2594HV
www.ti.com SNVS118C –DECEMBER 1999–REVISED APRIL 2013
When using the adjustable version, special care must be taken as to the location of the feedback resistors and
the associated wiring. Physically locate both resistors near the IC, and route the wiring away from the inductor,
especially an open core type of inductor. (See Application Information section for more information.)
Table 1. LM2594/LM2594HV 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 21 A. Use the inductor selection guide for the 5V version shown in
Figure 22 or Figure 23. (Output voltages of 3.3V, 5V, or 12V Figure 22.
respectively.) For all other voltages, see the design procedure for the
B. From the inductor value selection guide shown in Figure 22, 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 Table 5, go to the
Maximum Load Current line. Each region is identified by an
L20 line and choose an inductor part number from any of the four
inductance value and an inductor code (LXX).
manufacturers shown. (In most instance, both through hole and
C. Select an appropriate inductor from the four manufacturer's part surface mount inductors are available.)
numbers listed in Table 5.
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 section in Application Information
Resistance) electrolytic capacitors between 82 μF and 220 μF and section.
low ESR solid tantalum capacitors between 15 μF and 100 μF
B. From the quick design component selection table shown in
provide the best results. This capacitor should be located close to
Figure 24, locate the 5V output voltage section. In the load current
the IC using short capacitor leads and short copper traces. Do not
column, choose the load current line that is closest to the current
use capacitors larger than 220 μF.
needed in your application, for this example, use the 0.5A line. In the
For additional information, see OUTPUT CAPACITOR section in maximum input voltage column, select the line that covers the input
Application Information section. voltage needed in your application, in this example, use the 15V line.
Continuing on this line are recommended inductors and capacitors
B. To simplify the capacitor selection procedure, refer to the quick
that will provide the best overall performance.
design component selection table shown in Figure 24. This table
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
the manufacturer's series that are listed in the table be used.
C. The capacitor voltage rating for electrolytic capacitors should be
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 120 μF 25V Panasonic HFQ Series
Simple™ version 4.1 or later.
120 μF 25V Nichicon PL Series
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 26 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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