LM2671MX-5.0 National Semiconductor, LM2671MX-5.0 Datasheet - Page 18

• Manufacturer Part Number: LM2671MX-5.0
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,BIPOLAR/MOS,SOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2671MX-5.0

• Rohs Compliant: NO

www.national.com
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
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
LM2671. 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 LM2671 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
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 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.
Style (Note 7)
SM and TH
SM and TH
PROCEDURE (Adjustable Output Voltage Version)
Case
IN
B
)
)
Voltage (V)
1.21–2.50
2.50–3.75
Output
OUT
22
IN
—
—
/V
pin.
IN
). The largest
33
—
—
18
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 1.2A) 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 × V
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 500 mA load, a capacitor with a RMS current rating
of at least 250 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 250 mA.
For a through hole design, a 68 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.
47
—
—
EXAMPLE (Adjustable Output Voltage Version)
Inductance (μH)
C1
68
—
IN
B
)
)
100
C1
C2
IN
) would be needed.
150
C2
C3
220
C3
C3