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LM2672LDX-3.3 데이터시트(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
부품명 LM2672LDX-3.3
상세설명  SIMPLE SWITCHER짰 Power Converter High Efficiency 1A Step-Down Voltage Regulator with Features
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제조업체  NSC [National Semiconductor (TI)]
홈페이지  http://www.national.com
Logo NSC - National Semiconductor (TI)

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LM2672 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version)
EXAMPLE (Fixed Output Voltage Version)
4. Input Capacitor (C
IN)
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 12 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
IN pin.
4. Input Capacitor (C
IN)
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
IN) would be needed.
The next higher capacitor voltage rating is 16V.
The RMS current rating requirement for the input capacitor in a
buck regulator is approximately 12 the DC load current. In this
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 16V line and note which capacitor values
have RMS current ratings greater than 500 mA.
For a through hole design, a 330 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
B)
This capacitor develops the necessary voltage to turn the switch
gate on fully. All applications should use a 0.01 µF, 50V ceramic
capacitor.
5. Boost Capacitor (C
B)
For this application, and all applications, use a 0.01 µF, 50V
ceramic capacitor.
6. Soft-Start Capacitor (C
SS - optional)
This capacitor controls the rate at which the device starts up.
The formula for the soft-start capacitor C
SS is:
6. Soft-Start Capacitor (C
SS - optional)
For this application, selecting a start-up time of 10 ms and using
the formula for C
SS results in a value of:
where:
I
SS
= Soft-Start Current
:4.5 µA typical.
t
SS
= Soft-Start Time
:Selected.
V
SSTH
= Soft-Start Threshold Voltage
:0.63V typical.
V
OUT
= Output Voltage
:Selected.
V
SCHOTTKY
= Schottky Diode Voltage Drop
:0.4V typical.
V
IN
= Input Voltage
:Selected.
If this feature is not desired, leave this pin open.
www.national.com
11


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