SNVS124C – NOVEMBER 1999 – REVISED APRIL 2013
PROCEDURE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A.
The catch diode current rating must be at least 1.3 times greater
A.
Refer to the table shown in
In this example, a 5A, 20V,
than the maximum load current. Also, if the power supply design 1N5823 Schottky diode will provide the best performance, and will
must withstand a continuous output short, the diode should have a not be overstressed even for a shorted output.
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
also provide good results. Ultra-fast recovery diodes typically have
reverse recovery times of 50 ns or less. Rectifiers such as the
1N5400 series are much too slow and should not be used.
4. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground pin to prevent large voltage
transients from appearing at the input. This capacitor should be
located close to the IC using short leads. 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.
For an aluminum electrolytic, the capacitor voltage rating should be
approximately 1.5 times the maximum input voltage. Caution must
be exercised if solid tantalum capacitors are used (see
on input capacitor). The tantalum capacitor voltage rating
should be 2 times the maximum input voltage and it is recommended
that they be surge current tested by the manufacturer.
Use caution when using ceramic capacitors for input bypassing,
because it may cause severe ringing at the V
IN
pin.
For additional information, see section on input capacitors in
section.
4. 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 12V, an aluminum electrolytic capacitor with a voltage
rating greater than 18V (1.5 × V
IN
) would be needed. The next
higher capacitor voltage rating is 25V.
The RMS current rating requirement for the input capacitor in 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 35V line and
note which capacitor values have RMS current ratings greater than
1.5A. A 680
μF/35V
capacitor could be used.
For a through hole design, a 680
μF/35V
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
on input capacitors in this
data sheet). The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
Table 1. LM2596 Fixed Voltage Quick Design Component Selection Table
Conditions
Output
Voltage
(V)
3.3
Load
Current
(A)
3
Max Input
Voltage
(V)
5
7
10
40
6
2
10
40
Inductor
Panasonic
HFQ Series
(μF/V)
470/25
560/35
680/35
560/35
470/25
330/35
330/35
Output Capacitor
Through Hole Electrolytic
Inductance
(μH)
Inductor
(#)
Nichicon
PL Series
(μF/V)
560/16
560/35
680/35
470/35
470/35
330/35
270/50
Surface Mount Tantalum
AVX TPS
Series
(μF/V)
330/6.3
330/6.3
330/6.3
330/6.3
330/6.3
330/6.3
220/10
Sprague
595D Series
(μF/V)
390/6.3
390/6.3
390/6.3
390/6.3
390/6.3
390/6.3
330/10
22 L41
22 L41
22 L41
33 L40
22 L33
33 L32
47 L39
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