Datasheet

LM2598
SNVS125C MARCH 1998REVISED APRIL 2013
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Table 1. LM2598 Series Buck Regulator Design Procedure (Fixed Output) (continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
4. Input Capacitor (C
IN
) 4. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is needed The important parameters for the Input capacitor are the input
between the input pin and ground to prevent large voltage transients voltage rating and the RMS current rating. With a nominal input
from appearing at the input. In addition, the RMS current rating of voltage of 12V, an aluminum electrolytic capacitor with a voltage
the input capacitor should be selected to be at least ½ the DC load rating greater than 18V (1.5 × V
IN
) would be needed. The next
current. The capacitor manufacturers data sheet must be checked to higher capacitor voltage rating is 25V.
assure that this current rating is not exceeded. The curve shown in
The RMS current rating requirement for the input capacitor in a buck
Figure 33 shows typical RMS current ratings for several different
regulator is approximately ½ the DC load current. In this example,
aluminum electrolytic capacitor values.
with a 1A load, a capacitor with a RMS current rating of at least 500
This capacitor should be located close to the IC using short leads mA is needed. The curves shown in Figure 33 can be used to select
and the voltage rating should be approximately 1.5 times the an appropriate input capacitor. From the curves, locate the 25V line
maximum input voltage. and note which capacitor values have RMS current ratings greater
than 500 mA. Either a 180 μF or 220 μF, 25V capacitor could be
If solid tantalum input capacitors are used, it is recomended that they
used.
be surge current tested by the manufacturer.
For a through hole design, a 220 μF/25V electrolytic capacitor
Use caution when using ceramic capacitors for input bypassing,
(Panasonic HFQ series or Nichicon PL series or equivalent) would
because it may cause severe ringing at the V
IN
pin.
be adequate. other types or other manufacturers capacitors can be
For additional information, see section on input capacitors in
used provided the RMS ripple current ratings are adequate.
Application Information section.
For surface mount designs, solid tantalum capacitors are
recommended. The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
Table 2. LM2598 Fixed Voltage Quick Design Component Selection Table
Conditions Inductor Output Capacitor
Through Hole Electrolytic Surface Mount Tantalum
Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague
Voltage Current Voltage (μH) (#) HFQ Series PL Series Series 595D Series
(V) (A) (V) (μF/V) (μF/V) (μF/V) (μF/V)
3.3 1 5 22 L24 330/16 330/16 220/10 330/10
7 33 L23 270/25 270/25 220/10 270/10
10 47 L31 220/25 220/35 220/10 220/10
40 68 L30 180/35 220/35 220/10 180/10
6 47 L13 220/25 220/16 220/16 220/10
0.5 10 68 L21 150/35 150/25 100/16 150/16
40 100 L20 150/35 82/35 100/16 100/20
5 1 8 33 L28 330/16 330/16 220/10 270/10
10 47 L31 220/25 220/25 220/10 220/10
15 68 L30 180/35 180/35 220/10 150/16
40 100 L29 180/35 120/35 100/16 120/16
9 68 L21 180/16 180/16 220/10 150/16
0.5 20 150 L19 120/25 120/25 100/16 100/20
40 150 L19 100/25 100/25 68/20 68/25
12 1 15 47 L31 220/25 220/25 68/20 120/20
18 68 L30 180/35 120/25 68/20 120/20
30 150 L36 82/25 82/25 68/20 100/20
40 220 L35 82/25 82/25 68/20 68/25
15 68 L21 180/25 180/25 68/20 120/20
0.5 20 150 L19 82/25 82/25 68/20 100/20
40 330 L26 56/25 56/25 68/20 68/25
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