SNVS129E – MAY 2004 – REVISED JUNE 2005
PROCEDURE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A.
In normal operation, the average current of the catch diode is the
load current times the catch diode duty cycle, 1-D (D is the switch
duty cycle, which is approximately the output voltage divided by the
input voltage). The largest value of the catch diode average current
occurs at the maximum load current and maximum input voltage
(minimum D). For normal operation, the catch diode current rating
must be at least 1.3 times greater than its maximum average
current. However, 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 LM2675. The most stressful
condition for this diode is a shorted output condition.
B.
The reverse voltage rating of the diode should be at least 1.25
times the maximum input voltage.
C.
Because of their fast switching speed and low forward voltage
drop, Schottky diodes provide the best performance and efficiency.
This Schottky diode must be located close to the LM2675 using
short leads and short printed circuit traces.
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. 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 manufacturer data sheet must be checked to
assure that this current rating is not exceeded. The curves shown in
show typical RMS current ratings for several different
aluminum electrolytic capacitor values. A parallel connection of two
or more capacitors may be required to increase the total minimum
RMS current rating to suit the application requirements.
For an aluminum electrolytic capacitor, the voltage rating should be
at least 1.25 times the maximum input voltage. Caution must be
exercised if solid tantalum capacitors are used. The tantalum
capacitor voltage rating should be twice the maximum input voltage.
The tables in
show the recommended application voltage for
AVX TPS and Sprague 594D tantalum capacitors. It is also
recommended that they be surge current tested by the manufacturer.
The TPS series available from AVX, and the 593D and 594D series
from Sprague are all surge current tested. Another approach to
minimize the surge current stresses on the input capacitor is to add
a small inductor in series with the input supply line.
Use caution when using ceramic capacitors for input bypassing,
because it may cause severe ringing at the V
IN
pin.
5. Boost Capacitor (C
B
)
This capacitor develops the necessary voltage to turn the switch
gate on fully. All applications should use a 0.01
μF,
50V ceramic
capacitor.
EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A.
Refer to the table shown in
In this example, a 1A, 20V
Schottky diode will provide the best performance. If the circuit must
withstand a continuous shorted output, a higher current Schottky
diode is recommended.
4. Input Capacitor (C
IN
)
The important parameters for the input capacitor are the input
voltage rating and the RMS current rating. With a maximum input
voltage of 12V, an aluminum electrolytic capacitor with a voltage
rating greater than 15V (1.25 × V
IN
) would be needed. The next
higher capacitor voltage rating is 16V.
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 1A load, a capacitor with a RMS current rating of at least 500
mA is needed. The curves shown in
can be used to select
an appropriate input capacitor. From the curves, locate the 16V line
and note which capacitor values have RMS current ratings greater
than 500 mA.
For a through hole design, a 330
μF/16V
electrolytic capacitor
(Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
equivalent) would be adequate. Other types or other manufacturers'
capacitors can be used provided the RMS ripple current ratings are
adequate. Additionally, for a complete surface mount design,
electrolytic capacitors such as the Sanyo CV-C or CV-BS and the
Nichicon WF or UR and the NIC Components NACZ series could be
considered.
For surface mount designs, solid tantalum capacitors can be used,
but caution must be exercised with regard to the capacitor surge
current rating and voltage rating. In this example, checking
and the Sprague 594D series datasheet, a Sprague 594D 15
μF,
25V capacitor is adequate.
5. Boost Capacitor (C
B
)
For this application, and all applications, use a 0.01
μF,
50V ceramic
capacitor.
12
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