LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
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 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. In this example, a 5A,
20V, 1N5823 Schottky diode will provide the best perfor-
mance, 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 also provide good re-
sults. Ultra-fast recovery diodes typically have reverse recov-
ery times of 50 ns or less. Rectifiers such as the 1N5400
series are much too slow and should not be used.
4. Input Capacitor (CIN
)
4. Input Capacitor (CIN)
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
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 x VIN) would be needed.
The next higher capacitor voltage rating is 25V.
RMS current rating of the input capacitor should be selected to
1
be at least ⁄
2
the DC load current. The capacitor manufactur-
The RMS current rating requirement for the input capacitor in
ers 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.
1
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. The curves shown in Figure
13 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 an aluminum electrolytic, the capacitor voltage rating
should be approximately 1.5 times the maximum input volt-
age. Caution must be exercised if solid tantalum capacitors
are used (see Application Information on input capacitor). The
tantalum capacitor voltage rating should be 2 times the maxi-
mum input voltage and it is recommended that they be surge
current tested by the manufacturer.
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 ca-
pacitors can be used provided the RMS ripple current ratings
are adequate.
Use caution when using ceramic capacitors for input bypass-
ing, because it may cause severe ringing at the VIN pin.
For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the capaci-
tor surge current rating (see Application Information on input
capacitors in this data sheet). The TPS series available from
AVX, and the 593D series from Sprague are both surge cur-
rent tested.
For additional information, see section on input capaci-
tors in Application Information section.
11
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