LM2672N-12 National Semiconductor, LM2672N-12 Datasheet - Page 17

• Manufacturer Part Number: LM2672N-12
• Description: Voltage Regulator IC
• Manufacturer: National Semiconductor

Specifications of LM2672N-12

• Output Current: 1A
• Output Voltage: 12V
• No. Of Pins: 8
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Through Hole
• Package / Case: 8 Lead
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

4. 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 V
largest value of the catch diode average current occurs at the
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 greater than 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. The Schottky diode must be located close to
the LM2672 using short leads and short printed circuit traces.
5. Input Capacitor (C
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
data 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
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 Figure 15 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
6. Boost Capacitor (C
This capacitor develops the necessary voltage to turn the
switch gate on fully. All applications should use a 0.01 µF, 50V
ceramic capacitor.
If the soft-start and frequency synchronization features are
desired, look at steps 6 and 7 in the fixed output design
procedure.
LM2672 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
1
⁄
2
the DC load current. The capacitor manufacturer
IN
B
)
)
OUT
/V
IN
). The
IN
pin.
17
4. Catch Diode Selection (D1)
A. Refer to the table shown in Figure 12 . Schottky diodes
provide the best performance, and in this example a 1A, 40V
Schottky diode would be a good choice. If the circuit must
withstand a continuous shorted output, a higher current (at
least 2.2A) Schottky diode is recommended.
5. Input Capacitor (C
The important parameters for the input capacitor are the input
voltage rating and the RMS current rating. With a maximum
input voltage of 28V, an aluminum electrolytic capacitor with a
voltage rating of at least 35V (1.25 x V
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately
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
have RMS current ratings greater than 500 mA.
For a through hole design, a 330 µF/35V 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 Figure 15 , and the Sprague 594D series
datasheet, a Sprague 594D 15 µF, 50V capacitor is adequate.
6. Boost Capacitor (C
For this application, and all applications, use a 0.01 µF, 50V
ceramic capacitor.
EXAMPLE (Adjustable Output Voltage Version)
IN
B
)
)
1
⁄
2
the DC load current. In this
IN
) would be needed.
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