LM2672
LM2672 Series Buck Regulator Design Procedure (Fixed Output)
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 LM2672. 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
LM2672 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
1
⁄
2
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.
(Continued)
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 x 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
1
⁄
2
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
)
5. Boost Capacitor (C
B
)
This capacitor develops the necessary voltage to turn the switch For this application, and all applications, use a 0.01 µF, 50V
gate on fully. All applications should use a 0.01 µF, 50V ceramic ceramic capacitor.
capacitor.
6. Soft-Start Capacitor (C
SS
- optional)
This capacitor controls the rate at which the device starts up.
The formula for the soft-start capacitor C
SS
is:
6. Soft-Start Capacitor (C
SS
- optional)
For this application, selecting a start-up time of 10 ms and using
the formula for C
SS
results in a value of:
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