LM2598S-ADJ National Semiconductor, LM2598S-ADJ Datasheet - Page 15

• Manufacturer Part Number: LM2598S-ADJ
• Description: SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator/ with Features
• Manufacturer: National Semiconductor

LM2598 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version)
6. Input Capacitor (C
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
transients from appearing at the input. In addition, the RMS
current rating of the input capacitor should be selected to be
at least
data sheet must be checked to assure that this current rating
is not exceeded. The curve shown in Figure 15 shows typical
RMS current ratings for several different aluminum electro-
lytic capacitor values.
This capacitor should be located close to the IC using short
leads and the voltage rating should be approximately 1.5
times the maximum input voltage.
If solid tantalum input capacitors are used, it is recomended
that they be surge current tested by the manufacturer.
Use caution when using a high dielectric constant ceramic
capacitor for input bypassing, because it may cause severe
ringing at the V
For additional information, see section on input capaci-
tor in application information section.
(Continued)
Voltage
Output
(V)
1.2
1 2
1 5
2 4
2 8
4
6
9
1
⁄
2
the DC load current. The capacitor manufacturers
HFQ Series
IN
Panasonic
Through Hole Electrolytic Output Capacitor
330/50
220/25
220/25
180/25
120/25
120/25
pin.
(µF/V)
82/35
82/50
IN
)
FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table
Nichicon PL
Series
330/50
220/25
220/25
180/25
120/25
120/25
(µF/V)
82/35
82/50
Feedforward
Capacitor
4.7 nF
3.3 nF
1.5 nF
1.5 nF
1.5 nF
1 nF
1 nF
0
15
EXAMPLE (Adjustable Output Voltage Version)
6. Input Capacitor (C
The important parameters for the Input capacitor are the in-
put voltage rating and the RMS current rating. With a nominal
input voltage of 28V, an aluminum electrolytic aluminum elec-
trolytic capacitor with a voltage rating greater than 42V (1.5 x
V
voltage rating is 50V, a 50V capacitor should be used. The
capacitor voltage rating of (1.5 x V
line, and can be modified somewhat if desired.
The RMS current rating requirement for the input capacitor of
a buck regulator is approximately
this example, with a 1A load, a capacitor with a RMS current
rating of at least 500 mA is needed.
The curves shown in Figure 15 can be used to select an ap-
propriate input capacitor. From the curves, locate the 50V
line and note which capacitor values have RMS current rat-
ings greater than 500 mA. Either a 100 µF or 120 µF, 50V ca-
pacitor could be used.
For a through hole design, a 120 µF/50V electrolytic capaci-
tor (Panasonic HFQ series or Nichicon PL series or equiva-
lent) would be adequate. Other types or other manufacturers
capacitors can be used provided the RMS ripple current rat-
ings are adequate.
For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the ca-
pacitor surge current rating (see Application Information or
input capacitors in this data sheet). The TPS series available
from AVX, and the 593D series from Sprague are both surge
current tested.
To further simplify the buck regulator design procedure, Na-
tional Semiconductor is making available computer design
software to be used with the Simple Switcher line ot switch-
ing regulators. Switchers Made Simple (version 4.2 or later)
is available on a 3
IN
) would be needed. Since the the next higher capacitor
AVX TPS
330/6.3
220/10
220/10
100/16
Series
(µF/V)
Surface Mount Tantalum Output Capacitor
68/20
68/20
33/25
10/35
1
⁄
2
" diskette for IBM compatible computers.
IN
595D Series
)
Sprague
330/6.3
220/10
220/10
180/16
120/20
100/20
(µF/V)
33/35
33/35
IN
1
⁄
) is a conservative guide-
2
the DC load current. In
Feedforward
Capacitor
220 pF
220 pF
4.7 nF
3.3 nF
1.5 nF
1.5 nF
1.5 nF
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0