SNVS124C – NOVEMBER 1999 – REVISED APRIL 2013
PROCEDURE (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.
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. Rectifiers such as the 1N4001 series are much too slow and
should not be used.
6. Input Capacitor (C
IN
)
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 ½ the DC load
current. The capacitor manufacturers data sheet must be checked to
assure that this current rating is not exceeded. The curve shown in
shows typical RMS current ratings for several different
aluminum electrolytic 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
IN
pin.
EXAMPLE (Adjustable Output Voltage Version)
5. Catch Diode Selection (D1)
A.
Refer to the table shown in
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.
6. Input Capacitor (C
IN
)
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 electrolytic
capacitor with a voltage rating greater than 42V (1.5 × V
IN
) would be
needed. Since the the next higher capacitor voltage rating is 50V, a
50V capacitor should be used. The capacitor voltage rating of (1.5 ×
V
IN
) is a conservative guideline, and can be modified somewhat if
desired.
The RMS current rating requirement for the input capacitor of a buck
regulator is approximately ½ 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.
The curves shown in
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.
For additional information, see section on input capacitors in
For a through hole design, a 680
μF/50V
electrolytic capacitor
(Panasonic HFQ series or Nichicon PL series or equivalent) would
section.
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 rting (see
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, Texas
Instruments 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 3½″
diskette for IBM compatible computers.
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
Table 2. Output Capacitor and Feedforward Capacitor Selection Table
Output
Voltage
(V)
2
4
6
14
Through Hole Output Capacitor
Panasonic
HFQ Series
(μF/V)
820/35
560/35
470/25
Product Folder Links:
Nichicon PL
Series
(μF/V)
820/35
470/35
470/25
Feedforward
Capacitor
33 nF
10 nF
3.3 nF
Surface Mount Output Capacitor
AVX TPS
Series
(μF/V)
330/6.3
330/6.3
220/10
Sprague
595D Series
(μF/V)
470/4
390/6.3
330/10
Feedforward
Capacitor
33 nF
10 nF
3.3 nF
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