Datasheet
LM2598
www.ti.com
SNVS125C –MARCH 1998–REVISED APRIL 2013
Table 1. LM2598 Series Buck Regulator Design Procedure (Fixed Output) (continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
B. To simplify the capacitor selection procedure, refer to the quick B. From the quick design component selection table shown in
design component selection table shown in Figure 28. This table Figure 28, locate the 5V output voltage section. In the load current
contains different input voltages, output voltages, and load currents, column, choose the load current line that is closest to the current
and lists various inductors and output capacitors that will provide the needed in your application, for this example, use the 1A line. In the
best design solutions. 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 capacitor
manufacturers. It is recommended that both the manufacturers 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.
220 μF 25V Panasonic HFQ Series
220 μF 25V Nichicon PL Series
C. The capacitor voltage rating for electrolytic capacitors should be C. For a 5V output, a capacitor voltage rating at least 7.5V or more
at least 1.5 times greater than the output voltage, and often much is needed. But, in this example, even a low ESR, switching grade,
higher voltage ratings are needed to satisfy the low ESR 220 μF 10V aluminum electrolytic capacitor would exhibit
requirements for low output ripple voltage approximately 225 mΩ of ESR (see the curve in Figure 34 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.
D. For computer aided design software, see Switchers Made
Simple™ (version 4.2 or later).
3. Catch Diode Selection (D1) 3. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3 times greater A. Refer to the table shown in Table 8. In this example, a 3A, 20V,
than the maximum load current. Also, if the power supply design 1N5820 Schottky diode will provide the best performance, and will
must withstand a continuous output short, the diode should have a not be overstressed even for a shorted output.
current rating equal to the maximum current limit of the LM2598. The
most stressful condition for this diode is an overload or shorted
output condition.
B. The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
C. This diode must be fast (short reverse recovery time) and must be
located close to the LM2598 using short leads and short printed
circuit traces. Because of their fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance and
efficiency, and should be the first choice, especially in low output
voltage applications. Ultra-fast recovery, or High-Efficiency rectifiers
also provide good results. Ultra-fast recovery diodes typically have
reverse recovery times of 50 ns or less. Rectifiers such as the
1N5400 series are much too slow and should not be used.
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