LM2598 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version)
4. Input Capacitor (CIN
EXAMPLE (Fixed Output Voltage Version)
)
4. Input Capacitor (CIN
)
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
The important parameters for the Input capacitor are the
input voltage rating and the RMS current rating. With a
nominal input voltage of 12V, an aluminum electrolytic ca-
pacitor with a voltage rating greater than 18V (1.5 x VIN
)
at least 1⁄
2
the DC load current. The capacitor manufacturers
would be needed. The next higher capacitor voltage rating is
25V.
data sheet must be checked to assure that this current rating
is not exceeded. The curve shown in Figure 16 shows typical
RMS current ratings for several different aluminum electro-
lytic capacitor values.
The RMS current rating requirement for the input capacitor in
1
a buck regulator is approximately ⁄
2
the DC load current. In
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 16 can be used to select an appropriate input capaci-
tor. From the curves, locate the 25V line and note which
capacitor values have RMS current ratings greater than 500
mA. Either a 180 µF or 220 µF, 25V capacitor could be used.
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.
For a through hole design, a 220 µF/25V 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
ratings are adequate.
Use caution when using ceramic capacitors for input bypass-
ing, because it may cause severe ringing at the VIN pin.
For additional information, see section on input capaci-
tors in 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.
Conditions
Load
Inductor
Output Capacitor
Through Hole Electrolytic
Surface Mount Tantalum
Output
Max Input Inductance Inductor
Panasonic
HFQ Series
(µF/V)
Nichicon
PL Series
(µF/V)
330/16
270/25
220/35
220/35
220/16
150/25
82/35
AVX TPS
Series
(µF/V)
220/10
220/10
220/10
220/10
220/16
100/16
100/16
220/10
220/10
220/10
100/16
220/10
100/16
68/20
Sprague
595D Series
(µF/V)
#
Voltage Current
Voltage
(V)
5
(µH)
( )
(V)
3.3
(A)
1
22
33
L24
L23
L31
L30
L13
L21
L20
L28
L31
L30
L29
L21
L19
L19
L31
L30
L36
L35
L21
L19
L26
330/16
270/25
220/25
180/35
220/25
150/35
150/35
330/16
220/25
180/35
180/35
180/16
120/25
100/25
220/25
180/35
82/25
330/10
270/10
220/10
180/10
220/10
150/16
100/20
270/10
220/10
150/16
120/16
150/16
100/20
68/25
7
10
40
6
47
68
47
0.5
1
10
40
8
68
100
33
5
330/16
220/25
180/35
120/35
180/16
120/25
100/25
220/25
120/25
82/25
10
15
40
9
47
68
100
68
0.5
1
20
40
15
18
30
40
15
20
40
150
150
47
12
68/20
120/20
120/20
100/20
68/25
68
68/20
150
220
68
68/20
82/25
82/25
68/20
180/25
82/25
180/25
82/25
68/20
120/20
100/20
68/25
0.5
150
330
68/20
56/25
56/25
68/20
FIGURE 2. LM2598 Fixed Voltage Quick Design Component Selection Table
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