LM2594/LM2594HV Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
PROCEDURE (Adjustable Output Voltage Version)
6. Input Capacitor (CIN
EXAMPLE (Adjustable Output Voltage Version)
6. 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 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
at least 1
⁄2
the DC load current. The capacitor manufacturers
VIN) would be needed. Since the the next higher capacitor
data sheet must be checked to assure that this current rating
is not exceeded. The curve shown in Figure 13 shows typical
RMS current ratings for several different aluminum electro-
lytic capacitor values.
voltage rating is 50V, a 50V capacitor should be used. The
capacitor voltage rating of (1.5 x VIN) is a conservative guide-
line, and can be modified somewhat if desired.
The RMS current rating requirement for the input capacitor of
1
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.
a buck regulator is approximately
⁄2 the DC load current. In
this example, with a 400 mA load, a capacitor with a RMS
current rating of at least 200 mA is needed.
If solid tantalum input capacitors are used, it is recomended
that they be surge current tested by the manufacturer.
The curves shown in Figure 13 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 200 mA. A 47 µF/50V low ESR electrolytic
capacitor capacitor is needed.
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 a through hole design, a 47 µF/50V electrolytic capacitor
(Panasonic HFQ series or Nichicon PL series or equivalent)
would be adequate. Other types or other manufacturers ca-
pacitors can be used provided the RMS ripple current ratings
are adequate.
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.
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 4.1 or later)
is available from National’s web site, www.national.com.
Output
Volt-
age
Through Hole Output Capacitor
Surface Mount Output Capacitor
Panasonic
Nichicon PL
Series
(µF/V)
220/25
180/25
82/25
Feedforward
AVX TPS
Series
(µF/V)
220/10
100/10
100/10
100/16
100/16
68/20
Sprague
595D Series
(µF/V)
Feedforward
Capacitor
HFQ Series
(µF/V)
220/25
180/25
82/25
Capacitor
(V)
1.2
4
0
220/10
120/10
120/10
100/16
100/16
100/20
15/35
0
4.7 nF
4.7 nF
3.3 nF
2.2 nF
1.5 nF
1 nF
4.7 nF
4.7 nF
3.3 nF
2.2 nF
1.5 nF
220 pF
220 pF
6
9
82/25
82/25
1 2
1 5
2 4
2 8
82/25
82/25
82/25
82/25
82/50
120/50
120/50
10/35
82/50
820 pF
10/35
15/35
FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table
www.national.com
14
NSC [ National Semiconductor ]
