LM2594/LM2594HV
LM2594/LM2594HV Series Buck Regulator Design Procedure (Fixed
Output)
(Continued)
PROCEDURE (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 LM2594. 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 LM2594 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 re-
verse recovery times of 50 ns or less. Rectifiers such as the
1N4001 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 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
1
⁄
2
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
Figure 13
shows typical
RMS current ratings for several different aluminum electro-
lytic 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 recom-
mended that they be surge current tested by the manufac-
turer.
Use caution when using ceramic capacitors for input bypass-
ing, because it may cause severe ringing at the V
IN
pin.
For additional information, see section on input capaci-
tors in Application Information section.
4. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the in-
put 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 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
1
⁄
2
the DC load current. In
this example, with a 400 mA load, a capacitor with a RMS
current rating of at least 200 mA is needed. The curves
shown in
Figure 13
can be used to select an appropriate in-
put capacitor. From the curves, locate the 25V line and note
which capacitor values have RMS current ratings greater
than 200 mA. Either a 47 µF or 68 µF, 25V capacitor could be
used.
For a through hole design, a 68 µF/25V 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 surface mount designs, solid tantalum capacitors are
recommended. The TPS series available from AVX, and the
593D series from Sprague are both surge current tested.
EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A.
Refer to the table shown in
Figure 11.
In this example, a
1A, 20V, 1N5817 Schottky diode will provide the best perfor-
mance, and will not be overstressed even for a shorted out-
put.
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