Datasheet

LM2598
www.ti.com SNVS125C –MARCH 1998–REVISED APRIL 2013
Table 3. LM2598 Series Buck Regulator Design Procedure (Adjustable Output) (continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
6. Input Capacitor (C
IN
) 6. 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 28V, an aluminum electrolytic aluminum electrolytic
the input capacitor should be selected to be at least ½ the DC load capacitor with a voltage rating greater than 42V (1.5 × V
IN
) would be
current. The capacitor manufacturers data sheet must be checked to needed. Since the the next higher capacitor voltage rating is 50V, a
assure that this current rating is not exceeded. The curve shown in 50V capacitor should be used. The capacitor voltage rating of (1.5 ×
Figure 33 shows typical RMS current ratings for several different V
IN
) is a conservative guideline, and can be modified somewhat if
aluminum electrolytic capacitor values. desired.
This capacitor should be located close to the IC using short leads The RMS current rating requirement for the input capacitor of a buck
and the voltage rating should be approximately 1.5 times the regulator is approximately ½ the DC load current. In this example,
maximum input voltage. with a 1A load, a capacitor with a RMS current rating of at least 500
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 33 can be used to select an appropriate
input capacitor. From the curves, locate the 50V line and note which
Use caution when using a high dielectric constant ceramic capacitor
capacitor values have RMS current ratings greater than 500 mA.
for input bypassing, because it may cause severe ringing at the V
IN
Either a 100 μF or 120 μF, 50V capacitor could be used.
pin.
For a through hole design, a 120 μF/50V electrolytic capacitor
(Panasonic HFQ series or Nichicon PL series or equivalent) would
be adequate. Other types or other manufacturers capacitors can be
used provided the RMS ripple current ratings are adequate.
For surface mount designs, solid tantalum capacitors can be used,
but caution must be exercised with regard to the capacitor surge
current rating (see Application Information or INPUT CAPACITOR in
this data sheet). The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
For additional information, see section on input capacitor in To further simplify the buck regulator design procedure, Texas
Application Information section. Instruments is making available computer design software to be
used with the Simple Switcher line ot switching regulators.
Switchers Made Simple (version 4.2 or later) is available on a 3½″
diskette for IBM compatible computers.
Table 4. Output Capacitor and Feedforward Capacitor Selection Table
Output Through Hole Electrolytic Output Capacitor Surface Mount Tantalum Output Capacitor
Voltage
Panasonic Nichicon PL AVX TPS Sprague
Feedforward Feedforward
(V)
HFQ Series Series Series 595D Series
Capacitor Capacitor
(μF/V) (μF/V) (μF/V) (μF/V)
1.2 330/50 330/50 0 330/6.3 330/6.3 0
4 220/25 220/25 4.7 nF 220/10 220/10 4.7 nF
6 220/25 220/25 3.3 nF 220/10 220/10 3.3 nF
9 180/25 180/25 1.5 nF 100/16 180/16 1.5 nF
1 2 120/25 120/25 1.5 nF 68/20 120/20 1.5 nF
1 5 120/25 120/25 1.5 nF 68/20 100/20 1.5 nF
2 4 82/35 82/35 1 nF 33/25 33/35 220 pF
2 8 82/50 82/50 1 nF 10/35 33/35 220 pF
Copyright © 1998–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LM2598