LM2675 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version)
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
EXAMPLE (Fixed Output Voltage Version)
The important parameters for the input capacitor are the input
voltage rating and the RMS current rating. With a maximum
transients from appearing at the input. This capacitor should be input voltage of 12V, an aluminum electrolytic capacitor with a
located close to the IC using short leads. In addition, the RMS voltage rating greater than 15V (1.25 x VIN) would be needed.
current rating of the input capacitor should be selected to be at The next higher capacitor voltage rating is 16V.
1
least ⁄
2
the DC load current. The capacitor manufacturer data
The RMS current rating requirement for the input capacitor in a
1
sheet must be checked to assure that this current rating is not
exceeded. The curves shown in Figure 14 show typical RMS
current ratings for several different aluminum electrolytic
capacitor values. A parallel connection of two or more
buck regulator is approximately
⁄ the DC load current. In this
2
example, with a 1A load, a capacitor with a RMS current rating
of at least 500 mA is needed. The curves shown in Figure 14
can be used to select an appropriate input capacitor. From the
capacitors may be required to increase the total minimum RMS curves, locate the 16V line and note which capacitor values
current rating to suit the application requirements. have RMS current ratings greater than 500 mA.
For an aluminum electrolytic capacitor, the voltage rating should For a through hole design, a 330 µF/16V electrolytic capacitor
be at least 1.25 times the maximum input voltage. Caution must (Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
be exercised if solid tantalum capacitors are used. The tantalum equivalent) would be adequate. Other types or other
capacitor voltage rating should be twice the maximum input
voltage. The tables in Figure 15 show the recommended
application voltage for AVX TPS and Sprague 594D tantalum
capacitors. It is also recommended that they be surge current
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
tested by the manufacturer. The TPS series available from AVX, Components NACZ series could be considered.
and the 593D and 594D series from Sprague are all surge For surface mount designs, solid tantalum capacitors can be
current tested. Another approach to minimize the surge current used, but caution must be exercised with regard to the capacitor
stresses on the input capacitor is to add a small inductor in
series with the input supply line.
surge current rating and voltage rating. In this example,
checking Figure 15, and the Sprague 594D series datasheet, a
Use caution when using ceramic capacitors for input bypassing, Sprague 594D 15 µF, 25V capacitor is adequate.
because it may cause severe ringing at the VIN pin.
5. Boost Capacitor (CB)
5. Boost Capacitor (CB)
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.
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