LM2671N-5.0 National Semiconductor, LM2671N-5.0 Datasheet - Page 11

• Manufacturer Part Number: LM2671N-5.0
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2671N-5.0

• Input Voltage: 40V
• Output Current: 500mA
• Output Voltage: 5V
• No. Of Pins: 8
• Mounting Type: Through Hole
• Voltage Regulator Type: Buck Switching
• Output Current Max: 0.5A
• Peak Reflow Compatible (260 C): No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

4. 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
5. 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.
6. Soft-Start Capacitor (C
This capacitor controls the rate at which the device starts up.
The formula for the soft-start capacitor C
where:
I
t
V
V
V
V
If this feature is not desired, leave this pin open.
SS
SS
LM2671 Series Buck Regulator Design Procedure (Fixed Output)
SSTH
OUT
SCHOTTKY
IN
= Soft-Start Current :4.5 µA typical.
= Soft-Start Time :Selected.
= Input Voltage :Selected.
= Output Voltage :Selected.
PROCEDURE (Fixed Output Voltage Version)
= Soft-Start Threshold Voltage :0.63V typical.
1
⁄
2
= Schottky Diode Voltage Drop :0.4V typical.
the DC load current. The capacitor manufacturer
IN
B
)
)
SS
- optional)
SS
is:
IN
pin.
11
4. 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 12V, an aluminum electrolytic capacitor with a
voltage rating greater than 15V (1.25 x V
The next higher capacitor voltage rating is 16V.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately
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 16V line and note which capacitor
values have RMS current ratings greater than 250 mA.
For a through hole design, a 100 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 Figure 15 , and the Sprague 594D series
datasheet, a Sprague 594D 15 µF, 25V capacitor is adequate.
5. Boost Capacitor (C
For this application, and all applications, use a 0.01 µF, 50V
ceramic capacitor.
6. Soft-Start Capacitor (C
For this application, selecting a start-up time of 10 ms and
using the formula for C
EXAMPLE (Fixed Output Voltage Version)
IN
B
SS
)
)
results in a value of:
SS
- optional)
1
⁄
2
the DC load current. In this
IN
) would be needed.
(Continued)
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