LM2675 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
4. Catch Diode Selection (D1)
EXAMPLE (Adjustable Output Voltage Version)
4. Catch Diode Selection (D1)
A. In normal operation, the average current of the catch diode is A. Refer to the table shown in Figure 12. Schottky diodes
the load current times the catch diode duty cycle, 1-D (D is the provide the best performance, and in this example a 1A, 40V
switch duty cycle, which is approximately VOUT/VIN). The largest Schottky diode would be a good choice. If the circuit must
value of the catch diode average current occurs at the maximum withstand a continuous shorted output, a higher current (at least
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 greater than the maximum current limit of the
LM2675. The most stressful condition for this diode is a shorted
output condition.
2.2A) Schottky diode is recommended.
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. The Schottky diode must be located close to the
LM2675 using short leads and short printed circuit traces.
5. Input Capacitor (CIN
)
5. Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
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 28V, an aluminum electrolytic capacitor with a
located close to the IC using short leads. In addition, the RMS voltage rating of at least 35V (1.25 x VIN) would be needed.
current rating of the input capacitor should be selected to be at The RMS current rating requirement for the input capacitor in a
1
1
least ⁄
2
the DC load current. The capacitor manufacturer data
buck regulator is approximately ⁄ the DC load current. In this
2
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
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
curves, locate the 35V line and note which capacitor values
capacitors may be required to increase the total minimum RMS have RMS current ratings greater than 500 mA.
current rating to suit the application requirements. For a through hole design, a 330 µF/35V electrolytic capacitor
For an aluminum electrolytic capacitor, the voltage rating should (Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or
be at least 1.25 times the maximum input voltage. Caution must equivalent) would be adequate. Other types or other
be exercised if solid tantalum capacitors are used. The tantalum manufacturers’ capacitors can be used provided the RMS ripple
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
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.
tested by the manufacturer. The TPS series available from AVX, For surface mount designs, solid tantalum capacitors can be
and the 593D and 594D series from Sprague are all surge used, but caution must be exercised with regard to the capacitor
current tested. Another approach to minimize the surge current surge current rating and voltage rating. In this example,
stresses on the input capacitor is to add a small inductor in
series with the input supply line.
checking Figure 15, and the Sprague 594D series datasheet, a
Sprague 594D 15 µF, 50V capacitor is adequate.
Use caution when using ceramic capacitors for input bypassing,
because it may cause severe ringing at the VIN pin.
6. Boost Capacitor (CB)
6. 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.
19
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