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LM2674 Datasheet(PDF) 20 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM2674
Description  SIMPLE SWITCHER Power Converter High Efficiency 500 mA Step-Down Voltage Regulator
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2674 Datasheet(HTML) 20 Page - National Semiconductor (TI)

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LM2674 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version)
EXAMPLE (Adjustable Output Voltage Version)
5. 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.
5. 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 28V, an aluminum electrolytic capacitor with a
voltage rating of at least 35V (1.25 x V
IN) would be needed.
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 500mA load, a capacitor with an 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
B)
6. 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.
For this application, and all applications, use a 0.01 µF, 50V
ceramic capacitor.
Case
Style (Note 7)
Output
Voltage (V)
Inductance (µH)
22
33
47
68
100
150
220
SM and TH
1.21–2.50
————
C1
C2
C3
SM and TH
2.50–3.75
C1
C2
C3
C3
SM and TH
3.75–5.0
C4
C5
C6
C6
C6
SM and TH
5.0–6.25
C4
C7
C6
C6
C6
C6
SM and TH
6.25–7.5
C8
C4
C7
C6
C6
C6
C6
SM and TH
7.5–10.0
C9
C10
C11
C12
C13
C13
C13
SM and TH
10.0–12.5
C14
C11
C12
C12
C13
C13
C13
SM and TH
12.5–15.0
C15
C16
C17
C17
C17
C17
C17
SM and TH
15.0–20.0
C18
C19
C20
C20
C20
C20
C20
SM and TH
20.0–30.0
C21
C22
C22
C22
C22
C22
C22
TH
30.0–37.0
C23
C24
C24
C25
C25
C25
C25
Note 7: SM - Surface Mount,
TH - Through Hole
FIGURE 16. Capacitor Code Selection Guide
www.national.com
20


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