LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
5. Catch Diode Selection (D1)
EXAMPLE (Adjustable Output Voltage Version)
5. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3 times
greater than the maximum load current. Also, if the power
supply design must withstand a continuous output short, the
diode should have a current rating equal to the maximum
current limit of the LM2596. The most stressful condition for
this diode is an overload or shorted output condition.
A. Refer to the table shown in Figure 11. Schottky diodes
provide the best performance, and in this example a 5A, 40V,
1N5825 Schottky diode would be a good choice. The 5A diode
rating is more than adequate and will not be overstressed
even for a shorted output.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time) and
must be located close to the LM2596 using short leads and
short printed circuit traces. Because of their fast switching
speed and low forward voltage drop, Schottky diodes provide
the best performance and efficiency, and should be the first
choice, especially in low output voltage applications. Ultra-fast
recovery, or High-Efficiency rectifiers are also a good choice,
but some types with an abrupt turn-off characteristic may
cause instability or EMl problems. Ultra-fast recovery diodes
typically have reverse recovery times of 50 ns or less. Recti-
fiers such as the 1N4001 series are much too slow and should
not be used.
6. Input Capacitor (CIN
)
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
The important parameters for the Input capacitor are the input
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
current rating of the input capacitor should be selected to be at
1
least
⁄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 electrolytic
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 3A load, a capacitor with a RMS current
rating of at least 1.5A 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
appropriate input capacitor. From the curves, locate the 50V
line and note which capacitor values have RMS current ratings
greater than 1.5A. Either a 470 µF or 680 µF, 50V capacitor
could be used.
Use caution when using a high dielectric constant ceramic
capacitor for input bypassing, because it may cause severe
ringing at the VIN pin.
For a through hole design, a 680 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 additional information, see section on input capaci-
tors in application information section.
For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the capaci-
tor surge current rting (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 cur-
rent 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 switching
regulators. Switchers Made Simple (version 4.3 or later) is
available on a 31⁄
" diskette for IBM compatible computers.
2
15
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